Etymology
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sine qua non 
"an indispensable condition," Latin, literally "without which not," from sine "without" (see sans) + qua ablative fem. singular of qui "which" (from PIE root *kwo-, stem of relative and interrogative pronouns) + non "not" (see non-). Feminine to agree with implied causa. The Latin phrase is common in Scholastic use. Sometimes a masculine form, sine quo non, is used when a person is intended. Proper plural is sine quibus non.
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right hand (n.)

the hand opposed to the left hand, late Old English rihthand; see right (adj.2) + hand (n.). So called as the one normally the stronger of the two. Applied to the right side generally by c. 1200. As a symbol of friendship or alliance, by 1590s. Figurative for "indispensable helper, person of use or importance," 1520s (right-hand man is attested by 1660s). Right-handed "having the right hand more useful than the left" is attested from late 14c.; as an adjective from c. 1700. Right-hander, of persons, "one who uses the right hand more skillfully than the left" is by 1885.

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