Nigh [ˈnī]
1. Near in place, time, or relationship; close by (archaic, poetic)
2. Nearly; almost (often followed by on or onto)
3. To approach (archaic)

● Usage:
Repent, the end is nigh!
He talked for nigh onto 2 hours.
He stopped talking, as the hour of his death was nighing.

First known use: before 10th century. Comes from Middle English nigh(e), neye.
As an adjective, nigh is an older form of the word "near" both of which are rooted in the Old English words nēah, nēh. It's most common to use nigh today when you're trying to sound poetic or referencing the archaic or biblical uses of the word.
Akin to Dutch na, German nahe, Old Norse nā-, Gothic nehw, nehwa.