preposition
1. : behind
a hall back of the main staircase — New Yorker
2. : beyond in past time : before
the history of the islands goes far back of this — Nathaniel Burt
two centuries back of the oldest complete Greek texts — I.M.Price
3. : in a hidden causal or background relation to
soon learned that the hardest kind of work was back of every success — Edward Bok
wondered what was back of his strange remark
4. : in support of
back of the legislation stood those likely to gain by its passage