只要
zhǐyào · HSK 2
so long as
只 (only) + 要 (need): 'only need' = so long as / provided
只 means only/just and 要 means need/want. Together 只要 literally reads as 'only need,' which maps cleanly onto the conditional 'so long as' or 'provided that' in English.
只 — mouth 口 + bird tail 八
Originally a pictograph showing a bird (口 as body) with its tail feathers (八) spread, meaning a single bird. It was later borrowed phonetically to write the grammatical particle and adverb meaning 'only' or 'just,' which became its dominant modern use.
要 — west 覀 (top) + woman 女 (bottom)
Originally a pictograph of a woman with hands on her waist, meaning 'waist' (the original form of 腰). The top 覀 is a stylized depiction of the waist area, not the character for 'west'. Over time it was borrowed for the meaning 'to want, important', so 腰 (with 月 flesh radical) was created for the original 'waist' sense.