I'd like to know if the word "side-street" in the following excerpt from the chapter Seven of The Great Gatsby is an equivalent of "side-street" in the sense of "underhanded," "secret," "sleazy":
“I found out what your ‘drug stores’ were.” He turned to us and spoke rapidly. “He and this Wolfsheim bought up a lot of side-street drug stores here and in Chicago and sold grain alcohol over the counter. That’s one of his little stunts. I picked him for a bootlegger the first time I saw him and I wasn’t far wrong.”
It is generally assumed by critics that Gatsby and Meyer Wolfsheim are engaged in bootlegging, illegal drug sales, and possibly gambling. When Gatsby's and Daisy's affair is being discussed openly, Tom uses this opportunity to interrogate Gatsby about these illegal business activities. Tom remarks that Gatsby and Wolfsheim had bought a number of "side-street" drugstores and they used them to sell grain alcohol over the counter. (This was during Prohibition in the United States; from 1920-1933, alcohol was banned, but was available through illegal means.)
"Side-street" outside of this context has no sleazy implications; it is simply an alley or a side street. Gatsby and Wolfsheim would not risk doing illegal business in the larger stores on main streets. It is easier to handle illegal drug trade in smaller, "out of the way" stores which are not as visible and therefore not as watched by police and law enforcements. However, one can interpret a sleaziness and an underhanded secrecy to these particular stores because they engage in illegal business. In other words, "side-street" means just that: stores on side-streets and alleys. What makes them sleazy, in this context, is that they have illegal business connections. Being on a side-street does not automatically make a business sleazy.
That being said, Tom's designation of 'drug stores' is in quotation marks which could indicate a note of sarcasm:
"I found out what your ‘drug-stores’ were.” He turned to us and spoke rapidly. “He and this Wolfshiem bought up a lot of side-street drug-stores here and in Chicago and sold grain alcohol over the counter. That’s one of his little stunts. I picked him for a bootlegger the first time I saw him, and I wasn’t far wrong.”
Again, although being on a side-street is not incriminating in and of itself, Tom's sarcasm and tone suggest that he is using "side-street" in a sarcastic or derogatory way. So, the answer to your question is that side-street doesn't necessarily mean "secret" or "sleazy." But in this context, Tom is indicating a secrecy, illegality, and sleaziness with the particular stores Gatsby is involved in.