Really means truly — in fact, as opposed to how it appears. Is that really true, what really happened, he really did it.
Its second job is emphasis, and that is much the commoner: really good, really tired, I really do not mind. On its own, really is a whole conversational move — an interested Really? invites more, and a flat Really. expresses doubt or disapproval, with the meaning carried entirely by the tone. Note not really, which is a soft no, and that really is stronger than very in speech but weaker in writing, where it often does nothing at all.
1adverbreallyTruly; in fact.
“Nobody knows what really happened that night.”
2adverbstrengthenUsed to strengthen what you are saying.
“It was really cold and nobody had a coat.”
Too few real examples of this word to estimate which sense is commonest.
really is a way of emphasising or asserting.
Middle English
really
English, today
really
From Middle English really, from und + real. From Middle English really, realy, rialliche, equivalent to real + -ly.
Etymology from Wiktionary, CC BY-SA