Etymology
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in fieri 
legal Latin, "in the process of being done," from fieri "to come into being, become," used as passive of facere "to make, do" (from PIE root *dhe- "to set, put").
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ab initio 
c. 1600, Latin, literally "from the beginning," from ab "from" (see ab-) + ablative of initium "entrance, beginning," which is from or related to the verb inire "to go into, enter upon, begin" (see initial).
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morituri te salutant 
Latin, literally "those about to die salute you," words addressed to emperor by gladiators upon entering the arena. Third person singular is moriturus te salutat, first person singular is moriturus te saluto.
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absit omen (interj.)
Latin, literally "may this omen be absent." Added to an expression of something one does not wish to be true or come true, "may it not be ominous;" from third person singular present subjunctive of abesse "be away" (see absent (adj.)) + omen (see omen).
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fieri facias (n.)
writ concerning a sum awarded in judgment (often requiring seizure and sale of property for debt), Latin, literally "cause it to be done, cause to be made," the first words of the writ, from Latin fieri "to be made, come into being" (see fiat). Second word from facere "to do" (from PIE root *dhe- "to set, put").
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Arbor Day 
day set aside in U.S. "for planting forest trees to make lumber for the generations yet to come" ["Congressional Record," June 1892], first celebrated April 10, 1872, in Nebraska (a largely treeless state), the brainchild of U.S. agriculturalist and journalist J. Sterling Morton (1832-1902). From Latin arbor, arboris "tree" (see arbor (n.2)).
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April fool (n.)

1680s; see April + fool (n.). April-gowk (from Old Norse gaukr "a cuckoo") is a northern variant. April Fool's Day customs of sending people on false errands seem to have come to England from France late 17c.; originally All Fool's Day (1712). In Cumberland, Westmorland and northern parts of Lancashire and Yorkshire, however, May 1 was the day for hoaxing, and the fool was a May gosling. That custom is attested by 1791.

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vice versa 

"the order being changed," c. 1600, Latin, from vice, ablative of vicis "a change, alternation, alternate order" (from PIE root *weik- (2) "to bend, to wind") + versa, feminine ablative singular of versus, past participle of vertere "to turn, turn about" (from PIE root *wer- (2) "to turn, bend"). "The phrase has the complete force of a proposition, being as much as to say that upon a transposition of antecedents the consequents are also transposed" [Century Dictionary].

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a priori 
1710, "from cause to effect," a Latin term in logic from c. 1300, in reference to reasoning from antecedent to consequent, based on causes and first principles, literally "from what comes first," from priori, ablative of prior "first" (see prior (adj.)). Opposed to a posteriori. Since c. 1840, based on Kant, used more loosely for "cognitions which, though they may come to us in experience, have their origin in the nature of the mind, and are independent of experience" [Century Dictionary]. Related: Apriorist; apriorism; aprioristic. The a is the usual form of Latin ab "off, of, away from" before consonants (see ab-).
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