Two Thumbs Up was born out of a need to express and extreme delight in a situation / person / movie / music / etc etc.

If something is good it gets a thumbs up.

if something is indescribably brilliant - it gets a 'Two Thumbs Up'

But it's simply not good enough to just say the phrase - you must also physically represent the Two Thumbs Up in a strong motion with both hands.

To be used sparingly.

A term used to reference cannabis/marijuana/weed.

An unofficial honor given for excellence in any field. 'Two thumbs up' is/are not contained to any profession/achievement, and, therefore, can be awarded for any outstanding accomplishment.

Putting both thumbs up both orifices of the lower female anatomy.

A sexual activity between three people, including at least two women, in which the third person is lying down on his back between two women who are kneeling up and supporting each other, while the person lying down uses his thumbs to massage the insides of the women's vaginal walls, thus causing both his thumbs to be in the upright position.

A brief expression of approval often used in sports between team mates or when someone is a clingy person and you want to exit the dcene.