Bagging someone?

No it doesnt mean taunting it means putting the person down/dismissing them/speaking ill of them/saying that they are .. " a useless piece of shit"

In the UK among teenagers, "bagging" someone means pulling sharply down on their trousers. A juvenile practical joke whose victims typically wear low slung jeans.

Bagging in the trousers sense was originally "debag."
>To remove someone’s trousers by force, a form of humiliation popular among boys and young men, often used as an initiation rite, or as a punishment for undesirable behaviour or to enforce group norms. Debagging also often occurs as friendly horseplay within a group, but the victim is usually a boy of lower status.
>Depriving a victim of his trousers strips him of his dignity and symbolically casts him out as unworthy to associate with other lads.
>The word originated among Oxford undergraduates in the early years of the 20th century and is derived from bags, slang for trousers (itself derived from the earlier and now obsolete bum-bags, i.e. bags to contain the bum (= bottom, arse, posterior). Bags in this sense is obsolescent, if not obsolete, used only jocularly or to create a period flavour. It survives only in debag and Oxford bags.
>The popularity of elastic-waisted trousers and the ease with which they can be pulled down has created a second meaning: to pull down someone’s trousers in a surprise attack from behind, more a minor embarrassment than a humiliation. source

It's "bagging" or "to bag out," apparently. I found:

Bagging is Australian slang for criticism.

to bag out (third-person singular simple present bags out, present participle bagging out, simple past and past participle bagged out)
(slang, Australian) to criticise someone
I don't mean to bag you out, but that top is really not flattering on you.

Australian slang meaning 'make fun of' or 'insult'. Other variations: bag(s) out, bagged out.
"You're always bagging me/him/her/them out!"

[A US reporter says that ] NBC's ratings are 'copping a bagging, as they say Down Under'. No, they aren't. A 'bagging' is a bad review, not a poor performance. It's entirely possible for a television program to be 'bagged' yet still rate highly. Like Survivor.

i think 'taking the piss' is a more familiar term, but in highschool if you were ridiculing someone we called it bagging them out - you notice someone trip down the stairs, and say to your friend "haha lets go bag him out".

Bag out - criticise sarcastically or harshly; knock

Cassell's dictionary of slang:

bag v. [1950s] (Aus/US) to denigrate, to criticize, thus +bagger+ a negative critic. [dial. +bag,+ to dismiss, to jilt/ ext. +bag v.1]

That "ext. bag v. 1" means that it is an extension of bag in the sense of "to shoot (to kill) of animals and humans" which dates to the 19th C.

Although Cassel calls this usage US, I've never heard it. However, Partridge gives
bag on, bag To insult someone in a competitive, quasi-freindly spirit, US

It looks like it may be black slang and relatively recent. One example Partridge gives is from a book about Hip Hop that gives "playing the dozens" as a synonym. (As defined by Wikipedia: The dozens is an element of the African American oral tradition in which two competitors, usually males, go head-to-head in a improvised competition of often good-natured, ribald trash talk. They take turns insulting—cracking, snapping, West Coast dissin'", or ranking on—one another, their adversary's mother or other family member until one of them has no comeback. This is called playing the dozens or doin' the dozens, and sometimes dirty dozens,')

Also from the free dictionary: To gain possession of; capture

This is how I've heard it used as in to have had sex with and/or gotten a date with. But it's been a really, really long time since I've heard it used in that way at all.

More recently I've only heard it on shows like Law and Order and CSI as in put the body in a bag.

To gain possession of; capture

I've heard that a lot, mostly in the context of hunting. He bagged a deer. I hope I'll bag some ducks.

But I've also heard it in other contexts. I bagged a CD player in perfect condition at the flea market.

Yup, that's how I've heard it more often - bagging stuff and animals, not people.

If I'm not mistaken "bags" is used in England the way "dibs" is used in the US. A child calls "bags" and thereby stakes a claim to something.