From The Collaborative International Dictionary of English v.0.48:

Molt Molt, Moult Moult, v. i. [imp. & p. p. Molted or Moulted; p. pr. & vb. n. Molting or Moulting.] [OE. mouten, L. mutare. See Mew to molt, and cf. Mute, v. t.] [The prevalent spelling is, perhaps, moult; but as the u has not been inserted in the otherwords of this class, as, bolt, colt, dolt, etc., it is desirable to complete the analogy by the spelling molt.] To shed or cast the hair, feathers, skin, horns, or the like, as an animal or a bird. --Bacon. [1913 Webster] Molt

From The Collaborative International Dictionary of English v.0.48:

U U ([=u]), the twenty-first letter of the English alphabet, is a cursive form of the letter V, with which it was formerly used interchangeably, both letters being then used both as vowels and consonants. U and V are now, however, differentiated, U being used only as a vowel or semivowel, and V only as a consonant. The true primary vowel sound of U, in Anglo-Saxon, was the sound which it still retains in most of the languages of Europe, that of long oo, as in tool, and short oo, as in wood, answering to the French ou in tour. Etymologically U is most closely related to o, y (vowel), w, and v; as in two, duet, dyad, twice; top, tuft; sop, sup; auspice, aviary. See V, also O and Y. [1913 Webster] See Guide to Pronunciation, [sect][sect] 130-144. [1913 Webster]

From WordNet (r) 2.0:

u adj : (chiefly British) of or appropriate to the upper classes especially in language use n 1: a nitrogen-containing base found in RNA (but not in DNA) and derived from pyrimidine; pairs with adenine [syn: uracil] 2: a heavy toxic silvery-white radioactive metallic element; occurs in many isotopes; used for nuclear fuels and nuclear weapons [syn: uranium, atomic number 92] 3: the 21st letter of the Roman alphabet

From Jargon File (4.3.1, 29 Jun 2001):

u- pref. Written shorthand for micro-; techspeak when applied to metric units, jargon when used otherwise. Derived from the Greek letter "mu", the first letter of "micro" (and which letter looks a lot like the English letter "u").