The phrase 'twice-a-week' is a modern English compound formed by combining common, ancient words. 'Twice' originates from Old English 'twiga' or 'twigea', meaning 'two times', itself from the Proto-Germanic 'twiz', related to the number two. 'Week' comes from Old English 'wice' or 'wicu', meaning 'a succession of days', from Proto-Germanic 'wikōn', likely referring to a change or turn. The indefinite article 'a' is a reduced form of Old English 'ān' (one). The logic of the compound is transparently arithmetic, directly stating a frequency of two occurrences per seven-day cycle, a concept that became standardized with the modern calendar.

This is a hyphenated compound phrase, not a single morpheme-based word. It is formed from the adverb 'twice', the indefinite article 'a', and the noun 'week'. Each component is an independent English word of Old English origin.