From Arabicلِ (li, “to, for, towards”). The initial syllable in suffixed forms is likely a contemporary development rather than influence from Arabicإِلَى (ʾilā, “to, for, towards”), whose initial glottal stop would have naturally been lost as seen below; instead, compare the development of the initial syllable in إِجَا (ʾija, “to come”).
Personal suffixes are attached to the stem ʾil-: إلي (ʾili, “to me”), إلك (ʾilak, “to you”), etc. Often, however, reduplicated forms from the stem laʾil- are used, thus لإلي (laʾili), لإلك (laʾilak).
The dative suffix is immediately followed by a second, personal suffix. When transcribing or imitating Levantine speech, the dative suffix is sometimes written separated by speakers of Arabic varieties where an analogous preposition does not act as a suffix, but it very much affects the base word in ways an unbound morpheme would not be able to do. In particular, it observes the following ruleset, where it's demonstrated along with the first-person suffix ـي (-i):
As expected of a newly-developed derivational suffix, it avoids creating a heavy syllable CVVC by contracting the long vowel if attaching to a hollow verb. In particular, long ā, even when raised to ē as in Lebanon and urban Syria, always shortens to a rather than to i — and North Levantine varieties have overwhelmingly merged short u and short i here into i.
جَاب (jāb (jēb), “he brought”) ⇒ جَبلِي(jabli, “he brought me”, literally “he brought to me”), not *جَابلِي(*jābli (*jēbli)).
شُوف (šūf, “look at, check out”, masculine imperative) ⇒ شِفلِي(šifli, “look at [...] for me, check out for me”)
Some speakers extend this process of contraction to the plural ending -īn of active participles.
قَايلِين (ʾāylīn, “have said”, pl) ⇒ قَايلِينلي(ʾāylīnli) or قَايلِنلي(ʾāylinli, “have told me”, plural, literally “have said to me”).
In order to avoid creating heavy syllables in all other contexts, it attaches as either -ill- or -all-. The -all- ending is used to attach to third-person masculine singular Form I biliteral verbs in the past tense, and the -ill- ending everywhere else.
حَطّ (ḥaṭṭ, “he set down”, transitive) ⇒ حَطَّلِّي(ḥaṭṭalli, “he set down for me”, transitive)
مشِيت (mšīt, “you walked”, masculine) ⇒ مشِيتِلَّك(mšītilli, “you walked to me; you walked for me”)
كَتَبت (katabt, “you wrote”, masculine) ⇒ كَتَبتِلِّي(katabtilli, “you wrote to me; you wrote for me”)
In other cases, i.e. in environments where naively sticking -l- onto the end of the base would not create a final heavy syllable, it attaches as just that.
كَتَبِت (katabit, “she wrote”) ⇒ كَتَبَِتلِي(katabatli, katabitli, “she wrote to me; she wrote for me”)
Unlike in Egyptian Arabic, the Levantine form of this suffix can only attach to the base word, not to any preceding suffixes. This causes it to shift object-pronoun suffixes onto the carrier يَاـ (yā-, yyā-). (And for speakers who don't use this carrier, the separated object-pronoun suffixes instead manifest as subject pronouns.)
Being dative in meaning, the suffix generally cannot express the possessive meaning of the unbound preposition above. However, the unbound form can occasionally be merged into a word it frequently collocates with, taking the same shape as the dative suffix. See صَار (ṣār).
The suffix is most-commonly seen attaching to verbs and their active participles, followed in commonness by elatives, then passive participles of verbs, and finally other parts of speech.
Its occurrence with other parts of speech, such as in the word بَعدِلّـ (baʿdill-, “(of time) remaining for”), can only result from the grammaticalization of an originally-unbound لَ(la). That particular term is composed of بَعْد (baʿd, “still; remaining”, adverb, literally “[there is] still [time]”) + لـ(l-, “for; belonging to”).
Suffixation to passive participles is also rare and often formed anew, which may nullify the historical merger of contracted ū into contracted ī. Two somewhat-common examples are مَسمُحلـ (masmuḥl-, “permitted for”) and مقَدَّرلـ (mʾaddarl-, “fated for, preordained for”).
It only attaches to elatives, in the context of a choice, that express a defining positive characteristic of a given option. For example, أَريَحَلِي (ʾaryaḥli, “more/most comfortable for me”) and أَنضَفلنَا (ʾanḍaflna, “cleaner/cleanest for us”) are both valid uses. To instead use it on an elative that describes a negative trait, such as *أَوْسَخلَك (*ʾawsaḵlak, “dirtier/dirtiest for you”) or *أصعَبلَك (ʾaṣʿablak, “more/most difficult for you”), is generally invalid, but will necessarily impart a positive slant if forced. Contrast to عَلَى (ʿala, “in relation to”), which an elative can similarly be construed with, but which does not require or carry any value judgment: أَهيَنعلَيك (ʾahyan ʿlēk, “easier for you”) is synonymous with أَهيَنلَك (ʾahyanlak, “easier for you”), but as indicated prior, the only proper way to construct an antonym is as أَصعَب علَيك(ʾaṣʿab ʿlēk, “more/most difficult for you”, literally “more/most difficult in relation to you”).
Its suffix-base form is lay-, similarly to عَلَى (ʿala). This distinguishes it from the preposition above when constructed with a personal suffix.
For the most part, only used in the phrases مِنُّو لَيه (minno lē, “in its entirety; altogether”, literally “from it to it”), قِدِر لَ(ʾidir la, “to be able to handle”, literally “to be able toward”), فِيه لَ(fī la, “can handle”, literally “can toward”), and إِجَا لَ(ʾija la, “to come for”). However, it is also uncommonly used to construe other verbs of motion as a generalized sense of the last phrase, where it retains the sense of intending “to apprehend or cause harm” suggested by come for:
وَاللَّه حَإِضهَر لَيك
waḷḷa ḥaʾiḍhar lēk
I swear, I'm going to come out there and come for you.
(literally, “I swear I'm going to come out toward you”)