'rugger' originates from English, specifically the word 'rugby', where 'Rugby' is the placename of the English town associated with the sport.

'rugger' formed in late 19th to early 20th century English as an informal derivative of 'rugby' by adding the agentive/slang suffix '-er', initially referring to a player and later also to the game.

Initially it meant 'a player of rugby,' and over time it also came to be used informally for 'the sport of rugby' (and occasionally for related items like a rugby shirt).