"Washing" is a common bit of jargon you'll encounter in laboratory work, and you can try and find a reference that can help you, we have several laboratory guides listed in the sticky topics at the top of many of our sub-forumns.
Like Borek: said, with more context, we may be able to talk you through what might be meant, depending on the context. Let's try: we usually wash something when its dirty, so what is the ammonia gas "dirty" with and what will take it away. Warning: for laboratory work, we don't have to "wash" with water, sometimes we wash with something else, like we wash a greasy flask with a solvent. Try to think along those lines.