(Sorry ’bout the wall of text, but this seems like a complicated one!)
tl;dr version: this seems like a sort of convergent evolution, two phrases coming from different roots but ending up close enough that they effectively gravitated together and merged as a single cliché. The key point: parting and Parthian each have earlier uses, quite distinct from each other, which convincingly lead up to parting/Parthian shot respectively.
The OED cites the figurative use of Parthian going back to c.1640, in the sense of fighting somewhat dishonestly, and in particular of attacking while retreating:
…To look upon this Parthian Fight / Of Love, still flying, or in chase, / Never encountering face to face. —E. Waller, Phillis, 1640.
But the association of this style with the Parthians predates the figurative use:
Or like the Parthian I shall flying fight. —Shakespeare, Cymbeline, 1518.
On the other hand, parting kiss and parting blow first appear in ?1570 and 1592 respectively, the latter used much like parting shot:
Thus much I must say for a parting blow. — R. Greene, Quip for Vpstart Courtier. 1592.
Similar uses of parting continue through the 17th and 18th centuries; the earliest use of Parthian they give that directly parallels it is in 1842:
They have probably enough dealt a Parthian shot to British interests, by setting the Nacional once more upon its legs. — The Times, April 20, 1842.
So it appears that parting blow was used in this sense significantly earlier; but also that Parthian was acquiring these associations independently, in e.g. the Cymbeline quote, well before the wording had converged enough that they would be likely to have influenced each other. But it looks to me as though the existing parting blow/shot/shaft/etc. might well have influenced the later appearances of Parthian blow, etc.
However, the OED’s etymologers themselves suggest an influence in the opposite direction, saying circumspectly (under the etymology of parting):
“In parting shot (see Compounds 1a) perhaps partly influenced by Parthian shot at Parthian adj. 2.”
As Cubbi’s and Mr. S & N’s references show, though, it’s possible to make arguments for influences in either direction, or for none at all.
A ‘line’ is a pick-up line here: something someone (usually a guy) says to someone (usually a girl) in order to get in their pants.
‘Pluck’ can sometimes be used as a euphemism for ‘fuck’; but it is also the verb used for playing the harp: you pluck the strings of the harp when you play it.
I’m not sure why that would mean that this line doesn’t work on harp players (it would be just as logical to me that harp players would be exactly where it would work)—but that’s not part of the joke. Which isn’t particularly funny, incidentally.
Best Answer
I'm 67 years old and use "yes" in this context somewhat frequently. It's not a joke, but a humorous response, and it's not restricted to young people or internet use at all. It means that the question itself is somewhat humorous, given the specific person being asked. It also means that the answer is larger or more of the characteristic being questioned than you would expect. The responder is going to be smiling when saying, "Yes!"
Example: Person 1: How much ice cream do you want? Person 2: Yes! (Start scooping and keep going until you fill up the bowl or I tell you to stop.)
Another Example: Person 1: Do you want a Jaguar or a Lamborghini for your free prize car? Person 2: Yes! (My desire for either is so great that I'll take either one.)