Thirty \Thir"ty\, a. [OE. thritty, AS. [thorn]r[=i]tig, [thorn]rittig; akin to D. dertig, G. dreissig, Icel. [thorn]rj[=a]t[=i]u, [thorn]rj[=a]tigi, [thorn]rir teger, Goth. [thorn]reis tigjus, i.e., three tens. See Three, and Ten, and cf. Thirteen.]

Being three times ten; consisting of one more than twenty-nine; twenty and ten; as, the month of June consists of thirty days.

Source: Webster's Revised Unabridged Dictionary (1913)
 

Thirty \Thir"ty\, n.; pl. Thirties.

1. The sum of three tens, or twenty and ten; thirty units or objects.

2. A symbol expressing thirty, as 30, or XXX.

Source: Webster's Revised Unabridged Dictionary (1913)