Uncool means not fashionable and not impressive. Trying, and visibly failing, to look easy about it.
The word depends entirely on cool, and it inherits all of that word's vagueness, since what is uncool changes by decade and by group. It has a second and more useful sense in speech, where saying that was uncool means it was unkind or out of order, and that use is a real reproach rather than a comment on style. It is informal throughout and belongs to conversation rather than writing.
Old English
cōl
Middle English
cool
English
cool
English, today
uncool
Built from
From un- + cool. cool comes from Middle English cool, from Old English cōl. From Middle English cool, from Old English cōl (“cool, cold, tranquil, calm”), from Proto-West Germanic kōl(ī), from Proto-Germanic kōluz (“cool”), from kalaną (“to be cold, to freeze”), Proto-Indo-European gel- (“to be cold, to freeze”). 2
Etymology from Wiktionary, CC BY-SA