From *εἴδω, a verb whose present tense is unknown, but is found only in εἶδον, the aorist principal part of ὁράω "I see", and οἶδα "I know". Ultimately from Proto-Indo-European *weyd- (“to see”).
εἶδος • (eîdos) (genitive εἴδεος or εἴδους); n, third declension;
- That which is seen: form, image, shape
- appearance, look
- fashion, sort, kind
- wares, goods
This inflection pattern originally had a sigma (i.e. *εἴδεσος), which was dropped early on in the history of Ancient Greek.
The irregular accentuation of this paradigm is explained by the fact that it is the contracted version of the above paradigm.