This is the 843rd most frequent Lithuanian word.
greičiausiai
"Most likely" or "probably."
In this sentence, "greičiausiai" is used as an adverb meaning "most likely," indicating a high probability of the action.
Grečiausiai jis atvyks rytoj.
Most likely, he will arrive tomorrow.
Here, "greičiausiai" functions as a superlative adverb meaning "the fastest," describing the manner of the action in comparison to others.
Jis bėga greičiausiai iš visų.
He runs the fastest of all.
In this usage, "greičiausiai" again acts as an adverb, suggesting a likelihood yet combined with certainty regarding the expected outcome.