pillage

Pilfer is a synonym of steal, but it typically implies a particular kind of stealing. What is pilfered is usually stolen stealthily—furtively, so that no one will notice—in small amounts and often again and again. One might, for example, pilfer cookies from a cookie jar until a plentiful supply has dwindled to nothing. The word is sometimes used for that kind of stealing: the stealthy and gradual stealing of something that isn't worth much anyway:

Money was tight enough that Dickey's family used silverware pilfered from the local Western Sizzlin….
— L. Jon Wertheim, Sports Illustrated 2 Apr. 2012

But it is also used when the stolen things are valuable indeed, and the act of pilfering a serious criminal act:

For generations, scavengers have prowled this city with impunity, pouncing on abandoned properties and light poles to pilfer steel, copper and other metals they could trade for cash at scrapyards. The practice left tens of thousands of buildings so damaged that they could not be restored, turning places like the North End into grim cityscapes that appeared to have been ravaged by a tornado.
— John Eligon, The New York Times, 15 Mar. 2015

Pilfer may remind one of a similar also-serious word: pillage. The two words share more than a first syllable; pilfer comes from an old word meaning "booty" (as in, things that are stolen or taken by force, especially during a war) and pillage means "to take things from a place by force especially during a war." But despite their similarities, the words in modern use are very different. Pilfer has long since shed the connotations of violence in its etymological past; what's pilfered is not taken violently. Pillage, on the other hand, remains firmly rooted in violence and especially war; it is not a term you apply when someone's been sneaking cookies from a cookie jar.