Webster's Revised Unabridged Dictionary · 1913

Y

Y (wī)

Y, the twenty-fifth letter of the English alphabet, at the beginning of a word or syllable, except when a prefix (see Y-), is usually a fricative vocal consonant; as a prefix, and usually in the middle or at the end of a syllable, it is a vowel. See Guide to Pronunciation, §§ 145, 178-9, 272.

It derives its form from the Latin Y, which is from the Greek U, originally the same letter as V. Etymologically, it is most nearly related to u, i, o, and j. g; as in full, fill, Anglo-Saxon fyllan; E. crypt, grotto; young, juvenile; day, Anglo-Saxon dag. See U, I, and J, G.

Y has been called the Pythagorean letter, because the Greek letter U was taken to represent the sacred triad, formed by the duad proceeding from the monad; and also because it represents the dividing of the paths of vice and virtue in the development of human life.

Y (wī) , noun

(Railroads) Something shaped like the letter Y; a forked piece resembling in form the letter Y.
(a)
(Railroads) One of the forked holders for supporting the telescope of a leveling instrument, or the axis of a theodolite; a wye.
(b)
(Railroads) A forked or bifurcated pipe fitting.
(c)
(Railroads) A portion of track consisting of two diverging tracks connected by a cross track.
Collocations (2)
Y level (Surveying) , an instrument for measuring differences of level by means of a telescope resting in Y's.
Y moth (Zoology) , a handsome European noctuid moth Plusia gamma) which has a bright, silvery mark, shaped like the letter Y, on each of the fore wings. Its larva, which is green with five dorsal white species, feeds on the cabbage, turnip, bean, etc. Called also gamma moth, and silver Y.

Y (ī) , pronoun

I. [Obsolete] — King Horn

Y-

[Old English y-, i-, Anglo-Saxon ge-, akin to Dutch & German ge-, Old High German gi-, ga-, Gothic ga-, and perhaps to Latin con-; originally meaning, together. Compare Com-, Aware, Enough, Handiwork, Ywis.]

A prefix of obscure meaning, originally used with verbs, adverbs, adjectives, nouns, and pronouns. In the Middle English period, it was little employed except with verbs, being chiefly used with past participles, though occasionally with the infinitive. Ycleped, or yclept, is perhaps the only word not entirely obsolete which shows this use.
That no wight mighte it see neither yheere. — Chaucer
Neither to ben yburied nor ybrent. — Chaucer

Also: I-