The story
A shortening of earlier no-see-um, a univerbation of no (“not, don't, cannot”) + see (“to perceive visually”) + um (alternative form of 'em, third-person singular/plural indicative pronoun).
Etymology from Wiktionary, CC BY-SA 4.0.
noseeum
no·see·umToday it means: A biting midge, a small flying insect that harasses people outdoors, particularly in warm weather after sunset. Full definition →