Means Let it go, man - it's not a big deal
Usage "He's wearing pink jeans!" "Allow it."
In the past, the Yoof of today have gone through a phase of getting "uptight", "het up" and "vexed" about things. But thankfully there is a chilled-out undercurrent in some corners of the urban sprawl, typified by the advocates of "allow it".
The "allow it" characters cool down even the most heated of youngsters with the calming rationalisation of this phrase. They point out that really life's too short and we should let go of the little things and accept them as they are. They are the cognitive behaviour therapists with the key words which, when repeated and embraced, de-stress the environment. A simple trick of the brain.
The "allow it, man" variety is the extra-chilled brand, extending the "allow it" with the endearing recognition of you as a person, "man".
So next time you see a Yoof getting wound up - in a riot perhaps - relax your voice and say, "Allow it, man." It may be that these calming words will leave them skipping off in the opposite direction.