der Schenkel
thigh
The upper part of a human or animal leg, located between the hip and the knee.
Nach der langen Wanderung schmerzten ihm die Schenkel.
After the long hike, his thighs hurt.

thigh / leg / side
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thigh
The upper part of a human or animal leg, located between the hip and the knee.
Nach der langen Wanderung schmerzten ihm die Schenkel.
After the long hike, his thighs hurt.

In German, a distinction is often made between 'Oberschenkel' (thigh, the part between hip and knee) and 'Unterschenkel' (lower leg, the part between knee and foot). When people just say 'Schenkel' (thigh/leg), they usually mean the thigh.
Er hat sich beim Fußball den Oberschenkel gezerrt.
He pulled his thigh muscle while playing soccer.
You can easily recognize the meaning of 'Schenkel' from the context. In connection with the body, sports, or pain, it refers to the leg. With food, it refers to meat; in mathematics, to geometry; and with objects, to their side parts.
Die beiden Schenkel des Winkels sind 5 cm lang.
The two sides of the angle are 5 cm long.
The idiom 'sich auf die Schenkel klopfen' means that someone laughs very loudly and heartily. It is a figurative expression for great amusement.
Der Witz war so gut, dass sich alle auf die Schenkel klopften.
The joke was so good that everyone slapped their thighs laughing.
In a culinary context, 'Schenkel' (leg) is almost always combined with the type of animal. Typical dishes are 'Hähnchenschenkel' (chicken leg), 'Putenschenkel' (turkey leg), or 'Lammschenkel' (leg of lamb).
Im Ofen braten schon die Putenschenkel für das Abendessen.
The turkey legs for dinner are already roasting in the oven.
page / side
'die Seite' and 'der Schenkel' can both refer to something on the side, but they are not used in the same way. 'Seite' is a general word for an area to the left or right of a center. 'Schenkel' is much more specific: in geometry, it means a line that forms an angle together with a second line, and with objects it can mean a lateral, arm-like part, for example on a pair of glasses. Learners may confuse the words when talking about shapes or objects. For normal directions, you say 'Seite'; 'Schenkel' only fits with certain body parts, shapes, or objects.

Beim Einkaufen ist ihr der Schenkel der Brille abgebrochen.
While shopping, the arm of her glasses broke off.
Nach dem Sport legt sie die Tasche auf die rechte Seite der Bank.
After exercising, she puts the bag on the right side of the bench.