A Leo Cullum cartoon from the New Yorker a while back, passed on by Christina Zable on Facebook:
That’s subtext as a verbing of the noun, parallel to the verbing text.
The now-common verbing of the noun text seems to be pretty recent. From the draft additions of March 2004 to the OED2 entry:
trans. Telecomm. To send (a text message) to a person, mobile phone, etc.; to send a text message to. Also intr.: to communicate by sending text messages.
with a first cite in 1998, from Usenet.
The noun subtext is much older; OED3 (June 2012) has the definition
An underlying and often distinct theme in a conversation, piece of writing, etc. Also in later use: spec. the subjective reality drawn on by a performer and underlying his or her interpretation of a role.
with a first cite:
1862 A. Crummell Future of Afr. x. 333 We find the curse upon Canaan repeated, that is, by implication, again and again, in this same chapter,..both in the context and sub-text.
The noun has become something of a vogue word — well, it’s obvious useful — and seems to have been recently verbed. Not yet in the OED, but here’s a representative use from a fiction-writing site:
Roughly speaking, subtexting refers to the art of putting a whole different layer of meaning under the surface, so that the dialogue is not really about what the dialogue appears to be about. [from the Advanced Fiction Writing site, quoted here]
But you don’t often see texting and subtexting put together.