[davem]

On 'Eleventy-one'

-ty (1)
suffix representing "ten" in cardinal numbers (sixty, seventy, etc.), from O.E. -tig, from a Gmc. root (cf. Du. -tig, O.Fris. -tich, O.N. -tigr, O.H.G. -zig, -zug, Ger. -zig) that existed as a distinct word in Gothic tigjus, O.N. tigir "tens, decades." English, like many other Germanic languages, retains traces of a base-12 number system. The most obvious instance is eleven and twelve which ought to be the first two numbers of the "teens" series. Their Old English forms, enleofan and twel(eo)f(an), are more transparent: "leave one" and "leave two." Old English also had hund endleofantig for "110" and hund twelftig for "120." One hundred was hund teantig. The -tig formation ran through 12 cycles, and could have bequeathed us numbers *eleventy ("110") and *twelfty ("120") had it endured, but already during the O.E. period it was being obscured. O.N. used hundrað for "120" and þusend for "1,200." Tvauhundrað was "240" and þriuhundrað was "360." Older Germanic legal texts distinguished a "common hundred" (100) from a "great hundred" (120). This duodecimal system, according to one authority, is "perhaps due to contact with Babylonia." (from the Online Etymology Dictionary)

So 'eleventy-one'. though it sounds twee, is a word like 'Dwarrow's' which Tolkien knew should have survived down into modern usage. The recent book 'Ring of Words' has a bit more on this, but unfortunately I'm at work now....