Based on the description, this lump is most likely a benign (non-cancerous) soft tissue lesion, especially given that it has been present for almost 10 years without any change in size, shape, or skin color. The most common possibilities in this situation are a lipoma, benign nerve-related swelling (neuroma), or a fibrous lesion.
The key point in your history is that the lump is painless at rest but causes sharp pain when pressed or hit, even lightly. This usually suggests that the swelling may be lying close to a nerve or containing nerve fibers, which is why even minor pressure produces significant pain. The absence of redness, skin changes, rapid growth, or systemic symptoms is very reassuring and makes serious conditions very unlikely.
Next Steps
In terms of management, there is no emergency since the lump has remained stable for many years. The next appropriate step would be a local ultrasound of the thigh, which can clearly identify whether the lump is fatty, fibrous, or nerve-related. This is a simple, non-invasive test and usually gives a clear diagnosis.
Treatment depends on symptoms rather than danger. If the lump is small, stable, and only occasionally painful, it can be safely observed. However, if the pain is bothersome or affecting daily activities, minor surgical removal is a definitive and curative option. Such procedures are usually straightforward when done electively.
Good luck! and do contact in case you need any surgical advice.