The idiomatic English phrase, “hand in glove,” refers to the collaboration of two parties, or a close or intimate relationship. Unlike many other English idioms, the exact meaning of this phrase is a subject of much debate. Generally, it can be used in the broadest sense to talk about co-operation or a close relationship, or in more specific senses, to address ideas of secrecy or discretion, as well as nefarious dealings or things that might not be quite legitimate.
While many dictionaries and other books define the phrase to be broadly signifying co-operation between individuals or groups, other resources show that English speakers often have specific ideas in mind when they use it. One of the best ways to illustrate this difference is by contrasting the phrase with another phrase, “hand in hand,” which does not have the distinct etymology of “hand in glove.” Doing something “hand in hand” simply means co-operating, so the addition of the more colorful word “glove” has led some to see the “hand in glove” phrase as a bit more sophisticated and specific in use.
Experts who contend that the phrase, “hand in glove,” has a broad definition say that it simply means the hand fits well in the glove, or that the relationship between the two parties fits well or works well. Others, though, see the use of the word “glove” in the idiom as signifying a secretive activity or relationship. For example, someone might say that a prominent person, “is working hand in glove,” with a mysterious or obscure party, as a way of drumming up speculation about possible collaborations.
It’s important not to confuse the phrase, “hand in glove,” with other phrases utilizing similar words. For example, the idiom, “iron fist in a velvet glove,” has a different meaning, though some might link it to the above one in terms of overall analogy. An “iron fist in a velvet glove” refers to a party taking aggressive action, but cloaking that action in a more subtle, gentler method, so that those being negatively affected do not perceive the full force of the actions taken against them. This phrase has somewhat fallen out of fashion, though it can still be used where appropriate. These other phrases reveal more about how someone might use the phrase, "hand in glove," to talk about a relationship between two parties.