'spare' originates from Old English, specifically the word 'spǣre' (or related Old English forms), where the root meant 'to refrain from using or to keep back.'

'spare' changed from Old English forms such as 'spǣre' through Middle English spellings like 'sparen'/'speren' and eventually became the modern English word 'spare', from which the adverb 'sparely' developed by adding the adverbial suffix '-ly'.

Initially, it meant 'to refrain from using or to refrain from harming (to show mercy or reserve)', but over time it evolved into its current senses including 'kept in reserve' and the adverbial sense 'in small quantity' expressed by 'sparely'.