The "prepend" operation is often called "unshift" when the "array" is 
used as a deque (ie. a data structure that can be used to implement both 
a stack and a queue).

The operation to add something (an "item") to a deque is often called 
"push". If you see the deque as a stack (First In, Last Out), you need 
an operation to get the the item that was pushed most recently, this 
operation is often called "pop".

Now if you see the deque as a queue (First In, First Out) you need an 
operation to get the item that was pushed least recently. This operation 
is called "shift" because if you have implemented the deque with a fixed 
origin it means shifting all the items except the first one one position 
towards this origin. Of course, this is not a very efficient of a deque 
used as a list...

Actually the real origin of the name "shift" is probably shift 
registers, which does something similar in hardware.

Oh, and unshift... I guess it's just an invented word for "the opposite 
of shift".

The most logical convention in that respect is probably the one used in 
the C++ STL: call push "push_back", unshift "push_front", pop "pop_back" 
and shift "pop_front".