Etymology
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be (v.)

Old English beon, beom, bion "be, exist, come to be, become, happen," from Proto-Germanic *biju- "I am, I will be." This "b-root" is from PIE root *bheue- "to be, exist, grow," and in addition to the words in English it yielded German present first and second person singular (bin, bist, from Old High German bim "I am," bist "thou art"), Latin perfective tenses of esse (fui "I was," etc.), Old Church Slavonic byti "be," Greek phu- "become," Old Irish bi'u "I am," Lithuanian būti "to be," Russian byt' "to be," etc.

The modern verb to be in its entirety represents the merger of two once-distinct verbs, the "b-root" represented by be and the am/was verb, which was itself a conglomerate. Roger Lass ("Old English") describes the verb as "a collection of semantically related paradigm fragments," while Weekley calls it "an accidental conglomeration from the different Old English dial[ect]s." It is the most irregular verb in Modern English and the most common. Collective in all Germanic languages, it has eight different forms in Modern English:

BE (infinitive, subjunctive, imperative); AM (present 1st person singular); ARE (present 2nd person singular and all plural); IS (present 3rd person singular); WAS (past 1st and 3rd persons singular); WERE (past 2nd person singular, all plural; subjunctive); BEING (progressive & present participle; gerund); BEEN (perfect participle).

The paradigm in Old English was: eom, beo (present 1st person singular); eart, bist (present 2nd person singular); is, bið (present 3rd person singular);  sind, sindon, beoð (present plural in all persons); wæs (past 1st and 3rd person singular); wære (past 2nd person singular); wæron (past plural in all persons); wære (singular subjunctive preterit); wæren (plural subjunctive preterit).

The "b-root" had no past tense in Old English, but often served as future tense of am/was. In 13c. it took the place of the infinitive, participle and imperative forms of am/was. Later its plural forms (we beth, ye ben, they be) became standard in Middle English and it made inroads into the singular (I be, thou beest, he beth), but forms of are claimed this turf in the 1500s and replaced be in the plural. For the origin and evolution of the am/was branches of this tangle, see am and was.

That but this blow Might be the be all, and the end all. ["Macbeth" I.vii.5]
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sick (v.)
"to chase, set upon" (as in command sick him!), 1845, dialectal variant of seek. Used as an imperative to incite a dog to attack a person or animal; hence "cause to pursue." Related: Sicked; sicking.
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sick (adj.)

"unwell," Old English seoc "ill, diseased, feeble, weak; corrupt; sad, troubled, deeply affected," from Proto-Germanic *seuka-, which is of uncertain origin. It is the general Germanic word (compare Old Norse sjukr, Danish syg, Old Saxon siok, Old Frisian siak, Middle Dutch siec, Dutch ziek, Old High German sioh, Gothic siuks "sick, ill"), but in German and Dutch displaced by krank "weak, slim," probably originally with a sense of "twisted, bent" (see crank (n.)).

Restricted meaning "having an inclination to vomit, affected with nausea" is from 1610s; sense of "tired or weary (of something), disgusted from satiety" is from 1590s; phrase sick and tired of is attested from 1783. Meaning "mentally twisted" in modern colloquial use is from 1955, a revival of the word in this sense from 1550s (sense of "spiritually or morally corrupt" was in Old English, which also had seocmod "infirm of mind"); sick joke is from 1958.

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sick (n.)
"those who are sick," Old English seoce, from sick (adj.).
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sick-bay (n.)
"forepart of a ship's main deck used as a hospital," 1580s, from sick (adj.) + bay (n.2), in the sense "forepart of a ship between decks, forward of the bitts, on either side," from the notion of a recessed space.
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be-in (n.)
"a public gathering of hippies" [OED], 1967, from be + in (adv.).
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car-sick (adj.)
also carsick, "dizzy and nauseated from the motion of an automobile," 1908, from car (n.) + sick (adj.). Earlier it was used in the sense of "sick from the motion of a railroad car." Related: Car-sickness.
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aegrotat (n.)
certificate that a student is ill, Latin, literally "he is sick," third person singular of aegrotare "to be sick," from aeger "sick."
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sicken (v.)
c. 1200, "to become ill," from sick (adj.) + -en (1). Transitive sense of "to make sick" is recorded from 1610s. Related: Sickened; sickening. The earlier verb was simply sick (Old English seocan) "to be ill, fall ill."
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