Overview of Folk Etymology

Folk etymology involves a change in the form or pronunciation of a word or phrase resulting from a mistaken assumption about its composition or meaning. Also called popular etymology.

G. Runblad and D.B. Kronenfeld identify two main groups of folk etymology, which they call Class I and Class II. "Class I contains folk-etymologies where some change has occurred, either in meaning or form, or both. Folk etymologies of the Class II type, on the other hand, do not usually change the meaning or form of the word, but function mainly as some popular, though false, etymological explanation of the word" (Lexicology, Semantics, and Lexicography, 2000). Class I is by far the more common type of folk etymology.

Connie Eble points out that folk etymology "applies mostly to foreign words, learned or old-fashioned words, scientific names, and place-names" (Slang and Sociability, 1996).

Examples and Observations

• "The process of altering otherwise incomprehensible words, in order to give them a semblance of meaning, is called folk, or popular, etymology. A product of ignorance, it nevertheless should not be underestimated as a factor of language history, for many familiar words owe their form to it. In kitty-corner, kitty is a jocular substitution for cater-. Cater-corner is an opaque compound, while kitty-corner (diagonally from) suggests the movement of a prowling cat. . . .
  "Stepmother, stepdaughter, and so forth suggest the derivation from step. Yet a stepchild is not one step removed from its natural parent; -step goes back to a word meaning 'bereaved.' Many people share Samuel Johnson's opinion that bonfire is 'a good fire,' from French bon, but it means 'bonefire.' Old bones were used as fuel down to the 1800s. The vowel o was shortened before -nf (a regular change before two consonants), and a native English word began to look half-French."
  (Anatoly Liberman, Word Origins: Etymology for Everyone. Oxford University Press, 2009)

Woodchuck and Cockroach

"Examples: Algonquian otchek 'a groundhog' became by folk etymology woodchuck; Spanish cucaracha became by folk etymology cockroach."
(Sol Steinmetz, Semantic Antics: How and Why Words Change Meanings. Random House, 2008) 

Female

"Historically, female, from Middle English femelle (from Old French femelle, a diminutive form of Latin femina 'woman/female'), is unrelated to male (Old French male/masle; Latin masculus ('little' man/male); but Middle English femelle was clearly remodeled into female based on the association with male (approximately the 14th century) (OED). The remodeling of female brought female and male into their current and apparently sense-related and asymmetric relationship (one that many of us, now, are going to some lengths to unmake."
(Gabriella Runblad and David B. Kronenfeld, "Folk-Etymology: Haphazard Perversion or Shrewd Analogy." Lexicology, Semantics, and Lexicography, ed. by Julie Coleman and Christian Kay. John Benjamins, 2000)

Bridegroom

"When people hear a foreign or unfamiliar word for the first time, they try to make sense of it by relating it to words they know well. They guess what it must mean--and often guess wrong. However, if enough people make the same wrong guess, the error can become part of the language. Such erroneous forms are called folk or popular etymologies.
"Bridegroom provides a good example. What has a groom got to do with getting married? Is he going to 'groom' the bride, in some way? Or perhaps he is responsible for horses to carry him and his bride off into the sunset? The true explanation is more prosaic. The Middle English form was bridgome, which goes back to Old English brydguma, from 'bride' + guma 'man.' However, gome died out during the Middle English period. By the 16th century its meaning was no longer apparent, and it came to be popularly replaced by a similar-sounding word, grome, 'serving lad.' This later developed the sense of 'servant having the care of horses,' which is the dominant sense today. But bridegroom never meant anything more than 'bride's man.'"
(David Crystal, The Cambridge Encyclopedia of the English Language. Cambridge University Press, 2003)

Etymology
From the German, Volksetymologie