Certainly no formal definition, and I agree that like most pejoratives it tends to be flexibly wielded by the user to make a particular point.

In the broadest sense, I think it simply refers to people who are standing at/near the gate area in an attempt to secure an earlier boarding position, but who aren't part of a line that is actively boarding. In the days of five marked lines for boarding groups I still saw the term used offhand to refer to people joining those lines substantially in advance of the start of boarding, and now that we're back to the "norm" of not enough lines for everyone and no clear protocol for where to go before boarding starts, it seems easily applicable to any number of people standing at the gate before boarding begins, with a varying degree of specificity and negativity depending on the speaker. The fact that some of these people aren't in the first group to board will naturally give use to some conflict and negative feeling as e.g. 1K/GS need to push through when called.

For myself, I mentally think of myself as "licing the gate" whenever I awkwardly stand near it waiting for boarding to begin.