Learn English – If you’re “balled up” why are you confused

The Urban Dictionary suggests plonker is a person habitually drunk on cheap wine , (plonk) and hence someone who is foolish or useless.

I don't think that's right. I believe plonker in this context is a slang term for penis (chiefly used in the term pull someone's plonker, attempt a mild deception). Slang terms for penis are routinely used as terms of abuse, and that is why plonker is used for this purpose.

Edit I've just had a chance to look in Green's Dictionary of Slang. He has three entries for plonker:-

Anything large or substantial (figurative usage of standard English plonk, to hit or strike with a plonking noise). Earliest reference 1861.

Also plonk, the penis, earliest reference c1920 in the phrase pull one's plonker

Also plonk, a general term of abuse, widely popularized by the BBC TV Series Only Fools and Horses, earliest reference 1959.

I am and you are are the Standard English forms. In other dialects, the same form is used for all persons and numbers, so it is unlikely that a speaker of a nonstandard dialect would use both I are and you is. It would normally be are throughout or is throughout. As the sociolinguist Peter Trudgill says here:

Standard English has irregular forms of the verb to be both in the present tense (am, is, are) and in the past (was, were). Many nonstandard dialects have the same form for all persons, such as I be, you be, he be, we be, they be, and I were, you were, he were, we were, they were.