What's the accurate meaning of "forget yourself"?

Question

I was looking for a translation of the German expression "bevor ich mich vergesse" (lit. before I forget myself) and looked up if the literal translation could be used. In German this expression conveys a threat or warning.

However, looking at several dictionaries confused me a lot as the dictionaries seems not to agree with each other.
The first two quotes I provide below get close to the German meaning, but unfortunately they don't provide any examples. All the other ones have a totally different meaning.

forget (oneself): To lose one's reserve, temper, or self-restraint.
forget oneself: to lose one's dignity, temper, or self-control

forget oneself: to forget one's manners or training. (Said in formal situations in reference to belching, bad table manners, and, in the case of very young children, pants-wetting.)

John, we are going out to dinner tonight. Please don't forget yourself.

forget yourself: to act boldly or arrogantly. Phrase used to humble a person.

I believe you forget yourself sir!

forget yourself: to behave in a way that is not socially acceptable

I'm forgetting myself. I haven't offered you a drink yet!

I'm pretty sure that each of those definitions is correct, but probably some of them are only true for certain dialects or set of dialects.

I think my question now is if you would hear or say "forget yourself", what does that expression convey? Which of the definitions above could be applied?
Is there a definition which is acceptable for most dialects, i.e. is any of those understood in most parts of the world?

Finally, can this expression used as a threat or warning as in German?
Examples:

Hau ab, bevor ich mich vergesse. (lit. Take a hike, before I forget myself.)
Verschwinde, oder ich vergesse mich gleich. (lit. Get lost, or I'll forget myself.)

Answer 1

I don't speak German, but from the examples you've given I would say that the English and German usages are similar, but not identical. In English, it's almost always a rebuke, rarely a warning. It refers to forgetting about social rules, and your place within them. The expression can be mild or strong depending on the tone of voice, the speaker and the context.

In your examples:

John, we are going out to dinner tonight. Please don't forget yourself.

A mild rebuke. John has behaved badly in the past, and the speaker is gently requesting that he control himself tonight. Depending on the tone of voice, this could be a desperate plea from someone who feels slightly inferior to John (and therefore doesn't want to use stronger language) or a patronising order from someone who feels superior to John to the extent of treating him like a child.

I believe you forget yourself sir!

A strong rebuke from a social equal or superior. The speaker is very angry, and yet is trying to stay within the social rules himself.

I'm forgetting myself. I haven't offered you a drink yet!

The speaker has caught himself out in a social error, and is at once acknowledging his mistake and apologising for it. It's a mild self-rebuke, although tone of voice and context will show how embarrassed the speaker feels over the faux pas.

As you can see, it's all about context. The expression is almost never used in isolation, so the context is usually plain. The third one is probably the most common, though.

Finally, can this expression used as a threat or warning as in German?

It can, but it's not common. In English, such a threat would usually be wrapped up in a few more words. ("If you don't stop that, I might have to forget I'm a gentleman.") Like the second example, above, the speaker is trying to stay within the rules himself, while making it clear that he finds the other person's behaviour unacceptable. However, as social rules erode, this usage is becoming increasingly old-fashioned.

Answer 2

Converting my comment to an answer since people did not notice my link

According to what I can find, the English usage is nearly identical to the German

Wiktionary forget oneself

(idiomatic) to lose one's dignity, temper, or self-control.  

• 2011 August 28, Vincent Hogan, “'Where Rovers go from here could be thrilling or terrifying'”, Irish Independent: You see, the easy thing would be for Rovers to forget themselves.

• 2005, Elbow, Forget Myself No I know I wont forget you but I'll forget myself if the city will forgive me. To become unmindful of one's own personality; to be lost in thought.

Google book search gives us some examples

Now, I'm sorry to say, but it's time to take you home before I forget myself and do something that would ruin this evening.

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Get out of this house before I forget myself and slap your face

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I want you to leave now before I forget myself and throw you bodily into the street.

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Now clear out before I forget myself, and tell the copper you're working for that we'll tear him in half if he interferes with us