Nervous means worried and unable to relax about something coming — an exam, an interview, a flight. It is the everyday word for anxious in a specific, short-term way.
Two constructions are both correct and mean different things: nervous about something is the worry, nervous of something is closer to afraid of it. British English uses both; American English mostly uses about. The medical sense is entirely separate — the nervous system is the body's network of nerves, and has nothing to do with worry, though the words come from the same place. A nervous breakdown is an informal term for a period of mental collapse, and it is not a clinical diagnosis.
1adjectivenervousWorried and unable to relax about something ahead.
“She was nervous about the interview all week.”
synonyms: anxious, apprehensive, on edge
2adjectiveeasilyEasily frightened by nature.
“He is a nervous driver on motorways.”
3adjectiverelatingRelating to the nerves of the body.
“The drug acts on the central nervous system.”
Too few real examples of this word to estimate which sense is commonest.
nervous is a state of worry.
Latin nervosus meant sinewy and vigorous, from nervus, a sinew or tendon. English used nervous to mean strong and well-strung for centuries; the modern sense of jumpy came from the eighteenth-century idea that a person's nerves could be over-strung, and the compliment turned into a complaint.
Proto-Indo-European
*(s)neh₁-
Proto-Indo-European
*snéh₁wr̥ — sinew, tendon
Latin
nervōsus — nervous; sinewy; energetic, vigorous
Middle English
nervous — composed of or incorporating nerves
English, today
nervous
Built from
From Middle English nervous (“composed of or incorporating nerves”), from Latin nervōsus (“nervous; sinewy; energetic, vigorous”), from nervus (“nerve; muscle; sinew, tendon; (figuratively) energy, power; nerve; force, strength, vigour”) (ultimately from Proto-Indo-European snéh₁wr̥ (“sinew, tendon”)) + -ōsus. The English word is analysable as nerve + ous.
Etymology from Wiktionary, CC BY-SA