人：difference between ひと and -うと

Question

There are two readings of 人, as in these two examples: 恋人【こいびと】 and 素人【しろうと】

Why is 素人 (among others) the way it is and not しろひと or しろびと?

Furthermore, 狩人 is かりゅうど. Why is the final mora voiced here?

Answer 1

This phenomenon is called onbin “euphonic sound change”, specifically u-onbin, ウ音便, and in this case consists of ひと → うと (hito → uto), or びと → うど (bito → udo) with rendaku, in some cases combined with vowel changes (these are the same you see in modern forms of historic kana spellings).

The basic native reading (kun'yomi) is hito (there’s also ri, as in [一]{ひと}[人]{り} hito-ri, [二]{ふた}[人]{り} futa-ri); with rendaku this may become bito.

In the case of [素人]{しろうと}, originally [白人]{しろひと} (white person, for “inexperieced”), this underwent しろひと → しろうと, shirohito → shirouto → shirōto (note that ou → ō is not reflected as a change in kana).

In the case of [狩人]{かりゅうど}, this underwent かりびと → かりうど → かりゅうど, karibito → kariudo → karyuudo → karyūdo – note the additional vowel change.

An example that includes a familiar sound change is [仲人]{なこうど} “matchmaker”, from なかびと → なかうど → なこうど (nakabito → nakaudo → nakoudo → nakōdo). This underwent the あう → おう (au → ou → ō) sound change you’re familiar with in [お早う]{おはよう} (from hayaku → hayau → hayou → hayō).

There are many more details at Wikipedia, both in English and Japanese!