From Middle Welsh -tit, from Proto-Brythonic *-tid, from Proto-Celtic *-tūss (stem *-tūt-; compare Old Irish -tu), from Proto-Indo-European *-tuHts (whence Latin -tūs and Gothic -𐌳𐌿𐌸𐍃 (-dūþs)).
/tɪd/ and /dɪd/ are variants of the same suffix. /tɪd/ (phonetically [tʰɪd]) is always spelt -tid whereas /dɪd/ would, in theory, be represented by -tid after an unvoiced fricative (phonetically [tɪd]) and by -did after other voiced sounds (phonetically [dɪd]). /dɪd/, however, does not at present occur in any Welsh words after an unvoiced fricative.
-did causes i-affection of internal vowels.
- R. J. Thomas, G. A. Bevan, P. J. Donovan, A. Hawke et al., editors (1950–present), “-tid”, in Geiriadur Prifysgol Cymru Online (in Welsh), University of Wales Centre for Advanced Welsh & Celtic Studies