Although it is often considered a stand-alone 副詞（ふくし） (adverb), 必（かなら）ずしも is a combination of the adverb 必（かなら）ず ‘invariably’, or ‘certainly’, and the 副助詞（ふくじょし） (adverbial particles) し and も. し and も are often combined in order to strengthen the meaning of the word before it. Due to this, 必（かならず）ずしも may be translated as something similar to ‘absolutely certainly’.
However, 必（かならず）ずしも is an expression that is only used in negative sentences, so is commonly translated as ‘not always (A)’, ‘not entirely (A)’, or ‘not necessarily (A)’. Despite this, the literal meaning is close to ‘not absolutely certainly (A)’. 必（かならず）ずしも is often used at the beginning of a phrase, before expressing something that is not 100% certain.

• 男（おとこ）の人（ひと）は必（かなら）ずしも強（つよ）いわけではない。
  Men are not necessarily always strong.
• 刑務所（けいむしょ）にいる人（ひと）たちは必（かなら）ずしも犯罪（はんざい）を犯（おか）したとは限（かぎ）らない。
  People in prison are not always in there because they have committed a crime.
• 女（おんな）の人（ひと）は必（かなら）ずしも買（か）い物（もの）が好（す）きだとは言（い）えない。
  Women do not necessarily love shopping.