"struggle awkwardly and impotently," especially when hampered somehow, 1590s, of uncertain origin, perhaps an alteration of founder, q.v., influenced by Dutch flodderen "to flop about," or native verbs in fl- expressing clumsy motion. Figurative use is from 1680s. Related: Floundered; floundering. As a noun, "act of struggling," by 1867.

"flatfish," c.1300, from Anglo-French floundre, Old North French flondre, from Old Norse flydhra, from Proto-Germanic *flunthrjo (cognates: Middle Low German vlundere, Danish flynder, Old Swedish flundra "flatfish"), suffixed and nasalized form of PIE *plat- "to spread" (cognate: Greek platys "flat, wide, broad;" see plaice).