DCS is the shorthand for Double Chin Snapchat, which you either send to -- or receive from -- a friend.
The truly brave will send a DCS to their whole story. The truly drunk may do this as well.
The beautiful thing about a direct DCS, however, is that when you receive one personally (i.e. not via a “story”), there is a 99% chance it was sent to you on purpose and with great care, as opposed to those that can be dangerously misconstrued as a personalized Snap — and thus mortifyingly reciprocated. (See: social media equivalent of the awkward, nope-wasn’t-waving-at-you wave.)
If you send/receive a sober DCS it means you’re telling/being told or asking/being asked the following:
We are very close.
I am hungover.
Is this what I look like always?
What about when I am sleeping?
What about while having sex?
Consider it the highest honor of friendship, and as such: promptly send one back, but never, ever take a screen shot.