I know it translates to driftwood in English, but parts don't seem to equal the whole. Grève seems to translate to strike, so it looks more like strike wood than driftwood. I have searched around the net and read one article where it said grève referred to a sandy area on the shore of the river Seine. Is it quite literally a phrase that developed from wood that washed up in that part of the river? Further research brings up a couple interesting parallels. The grève was also a place where the unemployed would gather looking for work which I think compares with a work strike, with idle people. You can also strike the sails and your ship would literally be adrift. I may be reaching a bit and a native speaker could clear it up easily.