Words out of context

Lernesperanto came up with another enigma today: subten - back. This does not seem to make sense, what word class is it? It looks like a particle of some sorts, as in I looked back. But still odd. Also, back is quite ambiguous:

I've hurt my back when picking up this heavy item.

Back in 1970, petrol was much cheaper.

The damage was at the back of the house.

He was sitting in the back seat of the car.

When I finally looked up 'subten' in the on-line dictionary, I found the solution:

subtenajxo - abutment

subteni - abet, support, sustain

So it was neither of the above, but instead to back, the verb meaning 'to support somebody', as in I backed the winning candidate in the election. Another nice illustration that words don't have meaning out of context!