A thought, concept, or image present in the mind that is of such importance that it must be written down on the thighs because no other writing surface is availible.
Thighdeas are generally written down according to importance, with the least important by the knee and the most important on the upper thighs.
People have thighdeas most often late at night or during school/work when there is nothing better to think about.
Most blacks do not have thighdeas as the skin tone is usually too dark for the ink to appear. Some Latinos and Asians have trouble thinking of thighdeas if their skin color is dark.
Thighdeas have become more popular in the 21st century than ever before. Some have even taken the concept of thighdeas to a whole new level by writing homework, grocery lists, birthday/anniversary dates.
Thighdeas are usually written in blue or black pen or even sharpie if the thighdea is important enough.
You are permitted to write thighdeas on your ass only if completely necessary. Thighdeas are NEVER permitted outside the area covered by correctly fitted cargo shorts.
Never shave thighs to make room for a thighdea. Thighdeas are not valid if written on shaved thighs.
When Steve Jobs thought of the iPhone, he wrote the thighdea on his right thigh and presented the idea to his co-workers the next day.