side-text
While texting a group of people you and one of the people in the group also have a private texting conversation
I side-texted Vin to give me excuse get out of Jay's group message to hangout.
side-texted
When you are engaged in a social media conversation (i.e. FB status) and one of the commenters texts you when they disagree with what is being said or have additional private information.
Mary side-texted me after she saw what Asia's response was to her Facebook status.