In oracle bone script, it was likely a phono-semantic compound (形聲 / 形声): semantic 凡 + phonetic inverted 由 (OC*lɯw), hence a form vaguely similar to 甲. It's also possible that the phonetic component is instead an ancient form of 鏃 (OC*ʔsoːɡ).
In bronze inscriptions, 凡 corrupted into 舟, and the bottom-right subcomponent of 甲 gradually corrupted into two lines 巜 (now written 刂). The upper-right subcomponent (vaguely similar to 口) was eventually stylized as (亼) and moved to the top of the character in bamboo slips. Eventually, 舟 was further corrupted into 月, arriving at the modern form 俞.
