Bent backward or downward in a direction opposite to normal.
This is a botanical term. See for example Retrorse sedge. (Not to plug sites...but I'll have to remember that one- because when I get stumped by what it means to have retrorse perigynia, I can scroll down and hover over the word 'perigynium' and there's a picture. I love plants, but consistently get lost on the sea of botanical terms.)
To me, the term seems agreeable to inventive non-botanical uses. For example, the phrase 'retrorse arm' seems to provide a clear but painful mental image.
Antrorse means bent forward or upward.
Etymology: 1800s, from Latin retroversus, bent backwards