A stemple is a form of wooden step used in mining

Mining

Mining is the extraction of valuable minerals or other geological materials from the earth, from an ore body, vein or seam. The term also includes the removal of soil. Materials recovered by mining include base metals, precious metals, iron, uranium, coal, diamonds, limestone, oil shale, rock...

, caving

Caving

Caving—also occasionally known as spelunking in the United States and potholing in the United Kingdom—is the recreational pastime of exploring wild cave systems...

 and mountaineering

Mountaineering

Mountaineering or mountain climbing is the sport, hobby or profession of hiking, skiing, and climbing mountains. While mountaineering began as attempts to reach the highest point of unclimbed mountains it has branched into specialisations that address different aspects of the mountain and consists...

, usually a wooden bar set between notches in rock walls for climbing purposes, often one of a series forming a ladder.

Webster's Dictionary

Webster's Dictionary

Webster's Dictionary refers to the line of dictionaries first developed by Noah Webster in the early 19th century, and also to numerous unrelated dictionaries that added Webster's name just to share his prestige. The term is a genericized trademark in the U.S.A...

 says of the word:

If not well maintained, stemples can rot and should not always be trusted.

The origin of the word is in the German

German language

German is a West Germanic language, related to and classified alongside English and Dutch. With an estimated 90 – 98 million native speakers, German is one of the world's major languages and is the most widely-spoken first language in the European Union....

Stempel, meaning a stamp or prop.