Etymology
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hippopotamus (n.)

omnivorous ungulate pachydermatous mammal of Africa, 1560s, from Late Latin hippopotamus, from Greek hippopotamus "riverhorse," an irregular formation from earlier ho hippos potamios "the horse of the river"), from hippos "horse" (from PIE root *ekwo- "horse") + adjective from potamos "river, rushing water" (see potamo-). Replaced Middle English ypotame (c. 1300), which is from the same source but deformed in Old French. Glossed in Old English as sæhengest. Translated as river-horse in Holland's Pliny (1601).

Ypotamos comen flyngynge. ... Grete bestes and griselich ["Kyng Alisaunder," c. 1300]

Related: Hippopotamic.

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*ekwo- 
Proto-Indo-European root meaning "horse." Perhaps related to *ōku- "swift."

It forms all or part of: alfalfa; Eohippus; equestrian; equine; equus; hippo-; hippocampus; Hippocratic; Hippocrene; hippocrepian; hippodrome; hippogriff; Hippolytus; hippopotamus; Philip; philippic; Philippines; Xanthippe.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit açva-, Avestan aspa-, Greek hippos, Latin equus, Old Irish ech, Old Church Slavonic ehu-, Old English eoh, Gothic aihwa- all meaning "horse."
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*pet- 

Also petə-, Proto-Indo-European root meaning "to rush, to fly." 

It forms all or part of: accipiter; appetence; appetite; apterous; apteryx; archaeopteryx; asymptote; centripetal; Coleoptera; compete; competent; eurypterid; feather; helicopter; hippopotamus; Hymenoptera; impetigo; impetuous; impetus; iopterous; Lepidoptera; ornithopter; panache; panne; pen (n.1) "writing implement;" pennon; peripeteia; perpetual; perpetuity; petition; petulance; petulant; pin; pinion; pinnacle; pinnate; pinniped; potamo-; potamology; propitiation; propitious; ptero-; pterodactyl; ptomaine; ptosis; repeat; symptom.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit pattram "wing, feather, leaf," patara- "flying, fleeting;" Hittite pittar "wing;" Greek piptein "to fall," potamos "river, rushing water," pteron, pteryx "feather, wing," ptilon "soft feathers, down, plume;" Latin petere "to attack, assail; seek, strive after; ask for, beg; demand, require," penna "feather, wing;" Old Norse fjöðr, Old English feðer "feather;" Old Church Slavonic pero "feather;" Old Welsh eterin "bird."

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nixie (n.)

"water fairy, water sprite," 1816 (introduced by Sir Walter Scott), from German Nixie, from Old High German nihhussa "water sprite," fem. of nihhus, from Proto-Germanic *nikwiz (source also of Old Norse nykr, Old English nicor "water spirit, water monster," also used to gloss hippopotamus), perhaps from PIE *neigw- "to wash" (source also of Sanskrit nenkti "washes," Greek nizo "I wash," Old Irish nigid "washes").

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behemoth (n.)

late 14c., huge biblical beast (Job xl.15), from Latin behemoth, from Hebrew b'hemoth, usually taken as plural of intensity of b'hemah "beast." But the Hebrew word is perhaps a folk etymology of Egyptian pehemau, literally "water-ox," the name for the hippopotamus. Used in modern English for any huge beast.

Long before Jumbo was dreamed of, a hippo was exhibited by George K. Bailey, who invented the tank on wheels now used so generally in the circuses. The beast was advertised as "the blood sweating Behemoth of Holy Writ," and he made several men rich. [Isaac F. Marcosson, "Sawdust and Gold Dust," in The Bookman, June 1910]
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