Unusually means more than is normal. Unusually warm, unusually quiet, unusually for him.
It compares against what is expected rather than against a fixed level, so an unusually warm January is still cold. Unusually for somebody, placed at the start of a sentence, flags that what follows is out of character for that person.
1adverbunusuallyMore than is normal.
“It was unusually quiet in the office because most people were on holiday.”
unusually is a degree.
Unusual plus -ly, from un- plus usual, from Latin usualis, from usus, use.
Middle English
usualli
English
usually
English, today
unusually
From unusual + -ly or un- + usually. usually comes from Middle English usualli, equivalent to usual + ly. Broadly displaced native ġewunelīċe (from which wontly).
Etymology from Wiktionary, CC BY-SA