Translation:I accept your respect.
With greetings the exact translation is not so important. All we need to know is how they greet each other. But for those who'd like to know the real meaning of these words: "Shikamoo" comes from "nashika miguu yako" = I touch / hold your feet; "Marahaba" comes from Syriac (Aramaic, Assyrian) origin and was used by the first Christians. Mar = master or God, Haba = love, hence "Marahaba" = God is love.
Actually I'm not sure about that... The Aramaic word Mar which is used by Christians of the Middle East as a sign of utmost respect (as in the name of various Christian saints like Mar Elias for the prophet Elija) is written with Alef, or a long vowel. This however is not the case in the word "marhaba".
The Arabic term "marhaba" is derived from the root r-h-b (or رحب) with perhaps THE most usual semitic prefix: ma- (or -م).
I agree - the previous answer wanted only "Thank you" and gave that as an correction, but now it has changed. The inconsistancy is also shown when "Hello" is accepted in translation for "shikamoo" but welcome - a term implicit of great respect in English - is not.
If the full phrase "Thank you for your respect" is considered the only correct translation, then that should be the only thing accepted or promted from the start.
So you would only say shikamoo to someone older (or obviously higher status) than you?
If I said shikamoo to someone who is my equal, would they think I was giving them a compliment (I think you know so much more than me), being insincere (no one talks to a peer like that so you must be sarcastic), wrong (oops, you made a mistake, you meant to say Asante), or just strange (huh?)?
I’m just trying to get a feeling for when I might use it.
So if I’m very sincerely and respectfully thanking someone (who knows a lot more than I do) on this forum for explaining something to me I can say: Shikamoo. and they might reply: Marahaba. ?
And can I use it in a bit more of an informal setting? Like “Danke” and “Bitte” are used as a pair of phrases in German? Or “Grazie” and “Prego” in Italian?
To begin with, the 'pair' shikamoo/marahaba is never used to mean 'thank you' (danke) or 'please' (bitte). The translation of marahaba given here 'thank you for your respect' is not a literal translation, but just one way to translate it,
Shikamoo is the proper way to greet someone who is higher in rank (for instance a student greeting a professor), older (children/young people greeting their older relatives or elderly people in general), etc. It is difficult to explain exactly in which situation you would greet somebody with a simple habari ya asubuhi, jioni njema, etc. or with shikamoo. As a foreigner it is probably best to use shikamoo in cases where you are not sure!
Since greetings in Swahili are usually very extensive and may continue for quite a while, one can always follow the shikamoo/marahaba exchange with another greeting like "Habari ya familia?* (how is your family) or "Habari za siku nyingi?", lit. 'news of many days?', i.e. 'long time no see', etc.