Caked on antiperspirant armpit stains that resist all attempts to come out in the wash can be dislodged with something acidic.

You might try soaking in undiluted vinegar over a couple of days.

But I’m impatient so I use a small amount of pool acid directly on the damp stains, enough to re-wet them. After a few minutes the stains turn into a mushy paste that you can get out with a brush and in the wash.

⚠️ Hydrochloric acid is powerful stuff, don’t touch it if you don’t know how to handle it safely.