Unholy means wicked, or describes an unnatural combination. An unholy alliance, an unholy mess, an unholy row.
The religious sense is straightforward. The commoner modern use is as an intensifier for something disorderly: an unholy racket, an unholy hour of the morning. An unholy alliance is a partnership between parties who ought to be opposed, and it is one of the most-used phrases in political comment. Note holy, unholiness; and that the compound is Old English and unchanged.
1adjectiveunholyWicked or unnatural.
“The gangsters formed an unholy alliance with corrupt police officers.”
2adjectiveintensifierUsed as an intensifier for something disorderly.
“The kids were making an unholy racket in the next room.”
unholy is a quality of wickedness or discord.
Old English unhalig is un- plus holy, and holy comes from the same root as whole and hale. Every Germanic language builds the word the same way.
Proto-Germanic
*unhailagaz
Old English
unhāliġ
Middle English
unholi
English, today
unholy
From Middle English unholi, unhaliȝ, from Old English unhāliġ, from Proto-Germanic unhailagaz, equivalent to un- + holy. Cognate with Scots unhaly, Dutch onheilig, German Low German unhillig, German unheilig, Danish uhellig, Swedish ohelig.
Etymology from Wiktionary, CC BY-SA