weak

Concise Oxford English Dictionary © 2008 Oxford University Press:
weak/wiːk/

▶adjective

• 1 lacking physical strength and energy.

• 2 liable to break or give way under pressure.

  ■ not convincing or forceful.

  ■ not secure, stable, or firmly established.

  ■ (of prices or a market) having a downward tendency.

• 3 lacking power, influence, or ability.

  ■ lacking intensity.

  ■ (of a liquid or solution) heavily diluted.

  ■ (of features) not strongly marked.

  ■ (of a syllable) unstressed.

• 4 Grammar denoting a class of verbs in Germanic languages that form the past tense and past participle by addition of a suffix (in English, typically -ed).

– phrases
the weaker sex dated women regarded collectively.
weak at the knees helpless with emotion.

– derivatives
weakish adjective.

– origin OE wāc ‘pliant, of little worth’, ‘not steadfast’, reinforced in ME by ON veikr, from a Gmc base meaning ‘yield, give way’.