flack

From Middle English flacken (“to palpitate, flutter”), from Old English *flaccian, from Proto-West Germanic *flakkōn, from Proto-Germanic *flakkōną (“to beat”), from Proto-Indo-European *pleḱ-, which could be related to Ancient Greek πλάζω (plázō, “to turn away from”).

Akin to Middle Dutch vlacken (“to flicker, flash, sparkle”), Danish flakke (“to wander”), Swedish flacka (“to rove, rove about, ramble”), Icelandic flakka (“to move”). Compare also Icelandic flaka (“to flap, hang loose”), Swedish flaxa (“to flap, flutter”).

Most likely flak.

Variant of flak.