‘Chugger’ is short for ‘charity mugger’. You know them, they’re the the irritatingly happy people in tabards who try to stop you on the high street to persuade you to sign up to a direct debit to the charity they represent.
I understand why charities use them: they have to get money wherever they can and this method works to the to the tune of £120 million a year. That said, I personally always try to avoid them. They are endemic to the town where I work and some days I have to dodge them (sometimes 2 or 3 on each side of the road), the guys giving out vouchers for a free self-defence class, and the Big Issue sellers.
If you do stop to talk, they can almost bully you into it. That’s a drastic way of putting it but it’s hard to refuse money to a good cause when someone asks you outright. It’s hard enough on the phone. I just object to people asking me for money in this fashion; it’s invasive.