Etymology
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ad lib 

also ad lib., 1811 as a musical instruction, shortened from Latin ad libitum "to (one's) pleasure, as much as one likes" (c. 1600), from ad "to" (see ad-) + libitum "pleasure," accusative of libere "to please" (see libido). As a noun from 1825; as a verb by 1915.

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Kyrie eleison 

early 13c., a Greek liturgical formula adopted untranslated into the Latin mass, literally "lord have mercy" (Psalms cxxii.3, Matthew xv.22, xvii.15, etc.). From kyrie, vocative of kyrios "lord, master" (see church (n.)) + eleeson, aorist imperative of eleo "I have pity on, show mercy to," from eleos "pity, mercy" (see alms). Hence, the corresponding part of a musical setting of the Mass or Anglican Communion.

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