假定
jiǎ dìng · HSK 7
to assume
假 (false/hypothetical) + 定 (to fix, settle) = fix something as hypothetical = to assume
假 here means hypothetical or provisional rather than fake, and 定 means to set or settle. Together they describe settling on a supposition for the sake of argument, hence to assume or suppose.
假 — person radical 亻 + 叚 (borrow)
假 combines the person radical 亻 with 叚, which originally meant 'to borrow' and provides the sound. The core idea is a person borrowing something, which extended to 'borrowed,' 'not real,' or 'fake.' From there it also developed the meaning 'leave/vacation' (time borrowed away from work).
定 — roof 宀 + 正 (correct/upright, phonetic)
Originally written with 宀 (roof) over 正 (upright/correct), suggesting a household settled and orderly under its roof. Over time 正 simplified to 㐬-like form at the bottom, and the meaning shifted from 'settled home' to the broader senses of 'fix, stable, decide.'