canst

The word 'canst' is a fossilized form from Middle English, directly descended from the Old English 'cannst,' which combined the preterite-present verb 'cunnan' (to know, to be able) with the second-person singular ending '-st.' This ending was a standard feature of English conjugation when the pronoun 'thou' was in common use, specifically marking informality, intimacy, or social inferiority. The verb 'can' itself has a deep Germanic lineage, related to concepts of mental power and knowledge, which broadened to denote ability or capacity. The full form 'canst' fell out of everyday use as English simplified its verb conjugations and the 'thou/thee' pronoun system was replaced by 'you' for both singular and plural address.

This is an archaic verb form. The base word 'can' is a Germanic modal verb of uncertain origin, possibly from the Proto-Germanic kunnaną, meaning "to know, to be able." The suffix '-st' is an archaic second-person singular verb ending from Old English '-est' or '-ast,' used specifically with the pronoun 'thou.'