'second-language' is a compound formed from the words 'second' and 'language'. 'second' originates from Old French 'second' and ultimately from Latin 'secundus', where 'secundus' meant 'following' or 'next'. 'language' originates from Old French 'langage', ultimately from Latin 'lingua', where 'lingua' meant 'tongue'.

'second' came into English via Old French 'second' from Latin 'secundus'; 'language' passed from Latin 'lingua' to Old French 'langage' and then into Middle English as 'language', eventually forming modern English compounds such as 'second-language'.

The components originally had senses like 'following/next' (for 'second') and 'tongue' (for 'language'); over time the compound came to be used in modern contexts to mean 'a language learned after one's first language' (i.e., a non-native language acquired later).