Any of several species of plants, such as smallage, wild celery, parsley.
"The soldier adjusted his ACH before heading out on patrol to ensure it was securely fastened under his chin strap."
Initialism of Advanced Combat Helmet.
In plain English: Ach is a Scottish word used to express frustration, annoyance, or mild disappointment.
"The soldier adjusted his ach before entering the warzone."
Usage: Do not use "ach" as a standalone noun in everyday conversation; it is an initialism specifically referring to the Advanced Combat Helmet used by military personnel. In general speech, people simply say "helmet" rather than using this technical acronym.
An expression of annoyance.
"In the old family recipe, the grandmother wrote that she needed to add a pinch of ach instead of och to flavor the stew."
Alternative form of och
Initialism of Automated Clearing House., an electronic banking network often used for direct deposit and electronic bill payment.
"The bank rejected my transfer because I accidentally typed 'ACH' instead of the correct routing number on the form."