誰かさん (だれかさん, darekasan) literally means “somebody-san”, where “誰か” (dareka) means “somebody” with the Japanese honorific “san” attached to it. People use this phrase to refer to someone in a sarcastic and “anonymous” way, by which everyone knows who is being alluded to. This is a good expression to use when you blame or mock your friends in a jocular or ironic manner.
In fact, you can also simply say “誰か” (dareka) to blame someone ironically, but adding the honorific term ‘san’ makes it sound more casual, sarcastic, and less serious.
As already mentioned, dareka-san is a noun phrase that consists of the two parts “誰か” (dareka) meaning “somebody/anybody” and “さん” (san), a Japanese honorific. The kanji character “誰” (dare) has another meaning “who”, as in “誰がこれを食べたの？” (darega kore wo tabeta no?) meaning “Who ate this?”
Oops, we’ve missed the train because dareka-san (somebody) came late.
Ex2 (Your mom talking to you)
I’ve heard Tanaka’s son studies well unlike dareka-san (you).
Although you may find it somewhat strange, the honorific term “さん” (san) is often put after “息子“ (musuko) meaning “son” or “娘” (musume) meaning “daughter”, when you refer to someone’s daughter/son in a polite way. Therefore, it’ better to say musuko-san or musume-san when you refer to a child of someone who is older or in a higher potion than you.
Synonyms in English: somebody- mentioning no names -, you-know-who
Related words and phrases: 誰 (who), 誰か (somebody/anybody), さん (-san, a Japanese honorific)