The "thumbs" portion of your query could refer to several popular trends that involve physical interaction with the legs or hands:
: Users post photos and videos to normalize cellulite, stretch marks, and skin texture as signs of a life well-lived. mature legs thumbs
While "mature legs thumbs" is not a single, unified viral trend, it likely refers to a combination of distinct social media phenomena centered on body positivity for older women and specific romantic or interactive gestures. The "thumbs" portion of your query could refer