'pet-allowed' originates from Modern English as a compound of 'pet' and 'allow'. 'pet' comes from Scots/early Modern English 'pet' meaning 'darling, favorite', and 'allow' comes from Old French 'alouer' (from Latin elements 'ad-' + 'laudare') where 'laudare' meant 'to praise' and later developed the sense 'to permit'.

'pet' developed in Early Modern English from Scots usage meaning 'favorite' and was used for an animal kept as a favorite. 'Allow' entered Middle English via Old French 'alouer' and Middle English 'allowen', evolving to the modern verb 'allow'. The compound form (e.g., notice or adjectival phrase) arose in modern usage as English commonly forms compounds like 'pet-friendly' or phrases like 'pets allowed'.

Individually, 'pet' originally meant 'favorite' and 'allow' originally had senses related to 'praise' before shifting to 'permit'; together as 'pet-allowed' the compound means 'pets are permitted' and is used as an adjectival descriptor or posted notice.