'avenant' originates from French, specifically the word 'avenant', where the element 'a-' (ultimately from Latin 'ad-') meant 'to/toward' and 'venire' meant 'to come'.

'avenant' changed from Latin 'advenire' ('to come to/arrive') into Old French forms related to 'avenir'/'avener' and later the modern French 'avenant', and was borrowed into English in later usage as 'avenant'.

Initially it carried the sense of 'coming toward' or 'arriving'; over time in French it developed senses of 'suitable/agreeable' and, in legal/administrative contexts, 'that which is added' (an amendment), which are the senses reflected in English borrowing.