Necessary means needed — something will not work without it. Take the necessary precautions. It is necessary to book in advance. Is that really necessary?
The construction is it is necessary to do something, or it is necessary that something happen, and English never says somebody is necessary to do something. Note the spelling, which is among the top handful of English misspellings: one c, two s's, and the old schoolroom mnemonic is a shirt has one collar and two sleeves. The noun is necessity, and necessarily is the adverb — commonly used in the negative, not necessarily, to soften a disagreement.
1adjectivenecessaryNeeded for something to happen or work.
“Take whatever tools are necessary.”
synonyms: required, essential, needed
Too few real examples of this word to estimate which sense is commonest.
necessary is a state of being needed.
Latin necessarius meant unavoidable, from necesse, which may be ne- plus cedere, not to yield or give way. A necessary thing is one that will not step aside for you, which is a rather good description of a necessity.
Latin
necessārius — unavoidable, inevitable, required
Old French
necessaire
Middle English
necessarye
English, today
necessary
Built from
From Middle English necessarye, from Old French necessaire, from Latin necessārius (“unavoidable, inevitable, required”), variant of necesse (“unavoidable, inevitable”), probably from ne or non cessus, from the perfect passive participle of cēdō (“yield; avoid, withdraw”); see cede. Older use as a noun in reference to an outhouse or lavatory under the influence of English and Latin necessārium, a medieval term for the place for monks’ “unavoidable” business, usually located behind or attached to monastic dormitories.
Etymology from Wiktionary, CC BY-SA