The full sentence of Bonan tagon is "Mi deziras al vi bonan tagon". Is there something similar for Fek' al tio, where some elements of the original sentence are removed?
I'd say there are two ways to look at it.
I'm now about to ramble about the subject for a while, but you can just scroll to the conclusion at the end if you don't want to read all that.
Firstly, as you probably know, the interjection "fek" comes from the word "feko", which means poo or feces. (In fact, the word comes directly from "feces". The C used to be pronounced as a K in Latin.)
Taking this literal meaning, you could say "fek' al tio" stands for "mi deziras fekon al tio" (I wish it [to have] poo). Wishing something or someone poo is of course based on the general consensus that poo is not a very pleasant thing to have. So you could say it more or less means "I wish it something unpleasant".
But for this to be grammatically correct, the expression "fek' al tio" should technically have been "fekon al tio", because technically you may only remove the ending of a word and replace it with an apostrophe if it ends in -o, not if it ends in -on. You could of course blame this slightly faulty grammar on the fact that it's slang. There's more slang that doesn't fully make sense, even in Esperanto. But there's another way to explain it.
For that, let's look a bit closer at "fek". The dictionary PIV defines "fek!" as:
Interj. de kolero, malestimo, indigno, malbonŝanco ks.
translated to English:
An interjection of anger, contempt, offense, bad luck, and similar.
In other words, it's something you say to express anger, frustration, or some other negative emotion.
Going by this definition, "fek al tio!" would make sense as an abbreviation of "mi diras fek al tio". In other words "I express my anger/frustration towards that."
Also note that the interjection "fek!" does not have an apostrophe in the dictionary. Interjections, such as "fi!" "aĥ!" and even "amen", don't have a word ending in Esperanto, so there's no need for the apostrophe to replace the ending. So in this analysis "fek al tio!" (technically without apostrophe) is actually completely grammatically correct.
So in short, I think "fek al tio" could stand for two things.
- Looking at it literally: "Mi deziras fekon al tio." = "I wish it poo", something that we all agree on is something unpleasant to have.
- Looking at "fek" simply as a curse word or an interjection of anger and frustration: "Mi diras fek al tio." = "I say 'fek' to that", in other words "I express my anger/frustration towards that."
I think what many people mean when they say "fek al tio" or "fek al vi" is closer to the second meaning, but I do expect there's often a couple stinky brown smudges of the first meaning mixed in. ;)
Just as you can say "ve al mi!", where "ve" is used as an interjection, you can say "fek al tio!" That is at least the way you can see understand it from Kiel Paroli Maldece en Esperanto.
As Reta Vortaro says, "al" simply means "Esprimi direkton per orientilo" - showing direction. A direct translation of that would be "Excrement on that!" (where the word could be a verb or a noun).
Grammatically, it doesn't have to be an abbreviation of a complete sentence, but of course it is possible to think of sentences that could be abbreviated to this:
"Feku al tio!"
"Ni fekas al tio!"
"Mi prenas ekskrementon kaj donas gxin al tio!"