'equal-strength' originates from English, formed by combining the adjective 'equal' and the noun 'strength'; 'equal' ultimately comes from Latin 'aequalis' meaning 'level, even', and 'strength' comes from Old English 'strengþu' meaning 'physical power'.

'equal' entered English via Old French (from Latin 'aequalis') and Middle English 'equel/egal'; 'strength' developed from Old English 'strengþu' (from Proto-Germanic roots) into modern English 'strength'. The compound 'equal-strength' is a modern English formation that joins these two elements.

Individually, 'equal' originally meant 'level' or 'even' and 'strength' meant 'force or power'; combined as 'equal-strength', the meaning has remained literal and straightforward: 'having the same strength'.