The name inject and the argument order comes from Smalltalk (Ruby is heavily inspired by it). In Smalltalk arguments are part of the message name:

collection inject: aValue into: aBlock

Ruby inject appears to be derived from Smalltalk #inject:into:

#(1 2 3 4 5 6 7) inject: 10 into: [ :sum :each | sum + each squared ].

or from your example:

someHash := myArray inject: (Dictionary new) into: [ :accumulator :item | ... ].

The way I remember the order is it reflects the assignment you'd do is a while loop, sum := sum + each.