One of the things I like to think about is when the same word or phrase is used to describe multiple things, and how those different definitions are used in different social groups.
For example, when I was a teenager, I was standing in a comic book shop, engaging in a conversation about this or that comic. This conversation included myself, an employee of the store, and a rather portly fellow. Suddenly, the store employee digressed with this statement, "I can always tell who the subs are." We followed his gaze to an attractive young woman walking into the store. I was pretty confused, and asked, "Wait... can you explain that?" He responded that he was talking about people with subscriptions. I laughed, and said, "OH! I thought you were talking about sexual submissives." The portly gentleman said, "And *I* thought you were talking about submarine sandwiches."
The above illustrates how easily miscommunication can happen, and how divergent language can be used even amongst those who not only share a common language, but a common dialect. I find this incredibly interesting, and often very amusing. I often have to laugh at myself when this sort of thing happens, as it points out that my mind is *constantly* in the gutter.
For example, when many people see "CBT", they think "Cognitive Behavioral Therapy". I've also been told that this can also stand for "Compulsory Basic Training" which is what is required in the UK before someone can drive a motorcycle. On the other hand, when I see that, I usually think "Cock and Ball Torture". Nevermind the fact that I can NEVER see myself engaging in Cock and Ball Torture, it's the first thing that comes to mind when I see the letters "CBT".
Another example are the identifiers "Top" and "Bottom". These terms are both used within the gay male community, and within the BDSM community. Because I communicate regularly with people in both communities, I often have to catch myself to ensure that I'm using these terms in the correct way, depending on my audience. (A momentary digression: My therapist and I were talking about the above two examples, and he mentioned that he believes that "Top" and "Bottom" were re-appropriated from the gay community for use in the BDSM community. I mentioned that I'd been thinking about this for a while, and I agree with him. However, neither of us has proof that this is how this went down. I welcome any thoughts on the subject.)
Okay, so I'll give you a non-sex-related example to close. The word "fugue". As someone who has received a lot of training in music, for most of my life, I was only familiar with the musical term. In my early twenties, I became aware of the psychological term. What is really interesting about this, is that both of these terms actually trace back to the same word. But don't take my word for it, here's the etymology of both: 1590s, fuge, from It. fuga "ardor," lit. "flight," from L. fuga "act of fleeing," from fugere "to flee". Current spelling (1660s) is from the French version of the Italian word.
I really think it's interesting, and can tell you a lot about a person to discover which definitions of words they first thought up when hearing them. I invite anyone reading this to write their own thoughts on various words / terms that are commonly used with two different definitions, and which definition usually comes to mind first.