could

From Middle English coude, couthe, cuthe, from Old English cūþe, past indicative and past subjunctive form of cunnan (“to be able”) (compare related cūþ, whence English couth). Cognate with German konnte, Swedish kunde.

The -l- in the spelling was added in the early 16th century by analogy with should and would; this analogy formerly affected the pronunciation as well and was probably assisted by the tendency for /l/ to be lost in those words (and so not written, leading to shudd, wode, etc).