any idea what it means?
Can you give a little more context? Where/when did you hear? Can you use it in a sentence? Anything.
In Ireland and the UK “shell suits” was used to refer to shiney nylon tracksuits (the baggy kinds). That is the association I made for what it is worth.
Maybe discarded bottlecaps?
In Philadelphia real estate ads a “shell” is a derelict row house. You can buy them really cheap, gut what’s left of the inside, and remodel them.