As a Robin Hood enthusiast with a background in Linguistics, one of my favourite spurious folk etymologies is this hilariously clever one:

Giving the Finger

"Before the Battle of Agincourt in 1415, the French, anticipating victory over the English, proposed to cut off the middle finger of all captured English soldiers. Without the middle finger it would be
impossible to draw the renowned English longbow and therefore a soldier would be incapable of fighting in the future. This
famous weapon was made of the native English Yew tree, and the act of drawing the longbow was known as "plucking the yew" (or "pluck
yew"). Much to the bewilderment of the French, the
English won a major upset and began mocking the French by waving their middle finger at the defeated French, saying, "See, we can still
pluck yew! "PLUCK YEW!" Since 'pluck yew' is rather difficult to say, the difficult consonant cluster at the beginning has gradually changed to a labiodental fricative 'F', and thus the words often used
in conjunction with the one-finger-salute are mistakenly thought
to have something to do with an intimate encounter. It is also because of the pheasant feathers on the arrows used with the longbow that the symbolic gesture is known as "giving the bird".

Cheers, Hester
P.S. My archer friends tell me that one glaring error in the account is that English longbows were generally made of Spanish yew, not English yew, as the native variety had too many knots to make it suitable. And, as a linguist, I would point out that native English speakers have absolutely no difficulty pronouncing the consonant cluster "pl".