"struggle awkwardly and impotently," especially when hampered somehow, 1590s, of uncertain origin, perhaps from an alteration of founder (n.), 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 (source also of Middle Low German vlundere, Danish flynder, Old Swedish flundra "flatfish"), suffixed and nasalized form of PIE root *plat- "to spread."