'staid' originates from Middle English, ultimately from the verb 'stay' (Middle English staien / stauen), which traces back to Latin 'stare' where the root 'sta-' meant 'to stand' or 'to stay'.

'staid' developed from a past-participle/ adjective use in Middle English (e.g. Middle English staide/staid, meaning 'stayed' or 'steady') and gradually took on the fixed adjectival sense 'sedate, steady' in modern English.

Initially it meant something like 'stayed' or 'stable/standing in place', but over time it evolved to mean 'steady, sober, and conservative'—emphasizing temperament rather than literal staying.