Origins
The word 'is' — the third-person singular present tense of 'to be' — may be the most ancient essentially unchanged word in the English language. It descends from Old English 'is,' from Proto-Germanic *isti, from PIE *h₁ésti, the third-person singular form of the verb *h₁es- (to be, to exist).
The stability of this word across the Indo-European family is remarkable. PIE *h₁ésti is reflected as Sanskrit 'ásti' (is), Greek 'estí' (ἐστί, is), Latin 'est' (is), Old Church Slavonic 'jestŭ' (is), Lithuanian 'ẽsti' (is), Hittite 'ēšzi' (is), and German 'ist' (is). In each case, the word is phonologically close to the reconstructed PIE form — a degree of stability across six millennia and dozens of languages that is almost unparalleled in historical linguistics. The reason is that 'is' is the most frequently used verb form in these languages, and high-frequency words resist change because every generation of speakers hears and produces them thousands of times.
Proto-Indo-European Roots
The PIE root *h₁es- (to be) is the source of an enormous family of English words beyond the forms of 'be' itself. Through Latin 'esse' (to be), it produced 'essence' (the being of a thing), 'essential' (pertaining to being), 'absent' (being away — ab + esse), 'present' (being before — prae + esse), 'interest' (being between — inter + esse, originally a legal term for damages 'between' parties), and 'entity' (a being, from Late Latin 'entitās').
The English verb 'to be' is famously irregular — 'am, is, are, was, were, be, been, being' — and this irregularity reflects the fact that the modern paradigm is actually three separate PIE verbs fused together. 'Am/is' comes from PIE *h₁es- (to be). 'Be/been' comes from PIE *bʰuH- (to grow, to become). 'Was/were' comes from PIE *h₂wes- (to dwell, to stay). English speakers unknowingly conjugate three ancient verbs every time they use 'to be,' switching between them without any awareness that 'I am,' 'I was,' and 'I will be' are etymologically unrelated forms pressed into service as a single paradigm.