For a one-syllable word of little sparkle or fire, “dope” has an interesting variety of meanings.
Prominent over the last century or so is the use of “dope” to mean illegal drugs, typically marijuana or opioids, which tend to make people out of it, spacey, slow to react, or just plain silly.
This definition aligns with dopiness as a characteristic of personality, meaning someone who is mentally slow, lacking in awareness, and unable to catch on to conversation. That Disney’s 1937 version the Snow White tale included a dwarf named Dopey indicates how long we’ve been using the term this way.
Even now, when we rarely use “dope” to describe a person, everyone would easily understand what you meant if you called someone one.
Far more common now, though, is the slang use of “dope.” Like “bad,” “sick,” and “wicked” before it, “dope” has come to mean something cool. This probably relates to its use to describe illicit drugs, and it demonstrates the way slang tends to arise in communities who consider themselves outside of the mainstream, reversing meaning in order to establish counter-cultural credibility.
Perhaps out of the mainstream, the word is “dope” is also used to refer to chemicals that stiffen fabric. This use is common in reference to old tube-and-fabric aircraft, which used dope to help keep them aloft.
These chemicals will make you high in other ways as well, suggesting that maybe all dopes are inter-related after all.