'self-sacrificingly' is formed by combining the prefix 'self-' (from Old English 'self' meaning 'oneself'), the noun/verb 'sacrifice' (from Latin 'sacrificium'/'sacrificare'), the adjectival/participle suffix '-ing', and the adverbial suffix '-ly'.

'sacrifice' originates in Latin ('sacrificium' / 'sacrificare') and passed into Old French and Middle English as 'sacrifice'; compounds with 'self-' (e.g. 'self-sacrifice') developed in Modern English, yielding the adjective 'self-sacrificing' (present participle/adjectival use) and eventually the adverb 'self-sacrificingly'.

Originally 'sacrifice' had the sense 'to make sacred' or 'offer to a deity'; over time it shifted toward the broader meaning 'to give up something valuable', and 'self-sacrificingly' now means 'in a way that gives up one's own interests for others'.