ly Abbreviation of light year. Noun Abbreviation of light year. Synonyms: LY lightyear Forms lys Related psc pc Derived kly Mly Gly Verb Obsolete form of lie. One of the five large Isles, that ly near the coast […] - 1697, Christopher Irvine, Historiæ Scoticæ Nomenclatura Latino-vernacula, page 14: