'pets-allowed' originates from modern English as a compound of 'pet' and the past participle 'allowed' (from the verb 'allow'); 'pet' comes from Scots/English dialect 'pet' meaning 'favorite or tame animal', and 'allow' traces to Old French 'alouer' (to grant or permit).

'pets-allowed' changed from the simple two-word notice 'pets allowed' (a literal statement often seen on signs) and later became hyphenated and used attributively as an adjective in listings and adverts (e.g., 'a pets-allowed building').

Initially it was used as a full statement meaning 'pets are allowed'; over time it evolved into a compact adjectival label meaning 'permitting pets' used in descriptions and signs.