The Spanish word 'amistoso' (meaning 'friendly') has its roots in the Latin word 'amare' meaning 'to love'. From 'amare' came the Latin word 'amicus' meaning 'friend'. This evolved into the Vulgar Latin term '*amicitas' meaning 'friendship'. In Spanish, this developed into two related words: 'amigo' (friend) directly from Latin 'amicus', and 'amistad' (friendship) from Vulgar Latin '*amicitas'. Finally, 'amistoso' was formed by adding the Spanish suffix '-oso' (meaning 'full of') to 'amistad', literally meaning 'full of friendship' or 'friendly'.

'Amistoso' is closely related to two very common Spanish words that beginners often learn early: 'amigo' (friend) and 'amistad' (friendship). All these words share the same Latin root related to friendship and love. While 'amigo' refers to the person (a friend) and 'amistad' refers to the concept (friendship), 'amistoso' describes the quality of being friendly or showing friendship.

English speakers might recognize the connection to words like 'amicable' (friendly or peaceful) and 'amiable' (friendly and pleasant), which also come from the same Latin root 'amicus'. The English word 'amity', meaning friendship or peaceful relations, is also related. All these words preserve the core meaning of friendliness and peaceful relationships that was present in the original Latin.