pissant n.

"ant," late 14c., pisse-mire (early 14c. as a surname, Henricus pessemere), from pyss "urine" (said to be in reference to the acrid smell of an anthill) + mire "an ant" (mid-13c., early 13c. as a surname), perhaps from an unattested Old English word or from Old Norse maurr "ant" (cognate with Swedish myra, Danish myre, Middle Dutch miere, Dutch mier, Crimean Gothic miera "ant"), from PIE *morwi- (see Formica (2)). Compare pissant, also Old Frisian pis-imme, Norwegian migemaur (first element from Latin mingere); early Dutch mierseycke (with seycke "urine"), Finnish kusiainen (with kusi "urine").

He is as angry as a pissemyre,

Though þat he haue al that he kan desire.

[Chaucer]

Applied contemptuously to persons from 1560s.

c. 1500 shortening of Middle English ampte (late 14c.), from Old English æmette "ant," from West Germanic *emaitjon (source also of Old High German ameiza, German Ameise) from a compound of Germanic *e-, *ai- "off, away" + *mai- "cut," from PIE root *mai- (1) "to cut" (see maim). Thus the insect's name is, etymologically, "the biter-off."

As þycke as ameten crepeþ in an amete hulle [chronicle of Robert of Gloucester, 1297]

Emmet survived into 20c. as an alternative form. By a similar contraction, aunt "a parent's sister" is from Latin amita. White ant "termite" is from 1729. To have ants in one's pants "be nervous and fidgety" is from 1934, made current by a popular song; antsy embodies the same notion.