I had been talking with my Russian friend which used such words like "ебало", "ебаться" and "ебнутый". As I understood, this words have completely difference meaning. It is very curious.
Original meaning of еба́ть is to fuck. This root comes from Proto-Indo-European language where it had the same meaning (o̯i̯ebhoa̯ “I fuck”).
The root later acquired some developments into the meaning “to beat”, (въеба́ть, вы́ебать, means to beat somebody), possibly either because it is common to beat somebody into the reproductive organs or because fuck is associated with rape.
Further, еба́ло means a place where you can въеба́ть, i.e. beat. Since it is common to beat into the face, the word become to mean “a face that looks suitable to be beaten”.
Similarly, ёбнутый, ебану́тый means somebody hit so hard that his consciousness is damaged. That is mad.
Other words with the same root include
наеба́ть — to trick, cheat somebody;
отъеба́ться — to leave alone, leave in peace somebody;
заеба́ть — to annoy somebody out;
еба́шить — to blindly beat, blindly shoot at a place or object (in military sence), also do something without a break, also go, move blindly;
съеба́ть, съеба́ться — to flee;
проеба́ть — to lose something, for example, money, thing, opportunity;
подъеба́ть — to joke on somebody unpleasantly, provoke;
подъёбка — unfriendly joke, provocation;
доёбываться — to importune, to pester;
выёбываться — to epater, to show off, to strive to attract public attention with weird actions, to behave in an affected manner;
довыёбываться — to finish badly due to own strive to show off;
заеби́сь — well;
заебо́н — quirk, idiosyncrasy, kink; also a good thing;
недоёб — weird, aggressive, often misandric behavior of a woman due to the lack of sexual life.
There is a Russian word хлеба́ло wich is a vulgar way to say “mouth”. It is derived from the verb хлеба́ть, “to eat liquid food with a spoon; to drink”, this verb is colloquial and stylistically neutral. Xлеба́ло is formed by the non-productive suffix -л- which is used to form the names of instruments:
сиде́ть (to sit) — седло́ (saddle)
дыша́ть (to breathe) — ды́хало, дыха́ло (blowhole)
скрести́ (to scrape) — скребло́ (scraping knife)
ме́рить (to measure) — мери́ло (measure, criterion)
Since хлеба́ть rhimes with еба́ть, “to fuck”, so, by analogy, the word хлеба́ло got contaminated with еба́ть, and there appeared the word еба́ло, which is a still more vulgar way to say “mouth”. Later the meaning of the word got wider and it came to mean “face”, too, and now it means both.
Funny to say, but, although meaning “mouth”, from the etymological point of view the word еба́ло should have meant “penis, a thing to fuck with”. (-:
I'd like to point out that it is generally quite meaningless to ask "why" when to it comes to live languages, as they (languages) are the results of some "unorganized" collective authorship. Also, my apologies to the purists of Stack Exchange communities for stating this as an answer. I do realize that the most appropriate place for this will be a comment but it is not an option for me presently for the lack of reputation points.
More often than not, "eбало" means "mouth". An eloquent example of this usage is the wording
which is the most rude and aggressive Russian equivalent to "shut the fuck up"
(and which version I would recommend you never to use, except to provoke a fight).
This wording bears an allusion to filling up a hole in the ground (on this occasion, a metaphor for one's mouth).
On rare occasions, however, "ебало" can denote the face as a whole. Here is an excerpt from Igor Rasteryaev's song about provincial girls:
Тупое ебало, хуйню несу я.
Мой козырь единстванный -- жопа моя.