noun
shield; buckler
Literal protective shield used in combat or historical contexts.
騎士は左手に盾を持っていた。
The knight held a shield in his left hand.
noun
pretext; excuse; justification
Figurative use: something used as a cover or justification for an action, often in the phrase 盾に取る (to use as a pretext).
彼は病気を盾に欠席した。
He used illness as an excuse to be absent.
法律を盾に取って反論する。
To argue back using the law as a shield.
鎧 is armor worn on the body, while 盾 is a shield held in the hand.
The kanji 盾 is a pictograph of a shield. The reading たて is native Japanese, and the word has been used since Old Japanese.