I cannot think of a word that has undergone greater change during the course of my lifetime than the plain monosyllable "hack."
"Hack" entered my language, like much of my early vocabulary, through the medium of baseball. "He took a good hack at that fastball;" "he's up there hacking away." A hack was a hard swing, not necessarily a successful one. [There was also Stan Hack, the great Chicago Cub third baseman whose career was just coming to an end as I was becoming conscious.] In the days of my youth, "hack" was also slang for taxicab and a cab driver was a "hackie." I don't believe I've heard that particular use of the word in decades (except in 1940s movies). In seventh-grade "shop" class at PS 217, Mr. Kaminsky introduced me to the "hacksaw." The hack in hacksaw derives from a medieval word meaning "cut into pieces," but the taxi hack has an entirely dissimilar origin. It comes from hackney, a breed of horse and later the carriage itself that the horse drew -- the name of which then migrated to our big yellow taxi.
When did I first hear the expression, a "hacking cough"? And why hacking? Perhaps because the noise of a cough is similar to the sound of the short sharp blows when one hacks through a forest or the jungle with a machete. Maybe that's how it originated-- but I'm not entirely convinced.
Hack has another older meaning -- mediocre or failing, or not good at one's trade, as in a "hack writer," or a "political hack." This usage is probably a shortened version of "hackneyed," meaning trite or overused. When you waste time, you're just "hacking around." In such cases,"hack" denoted incompetence.
Nowadays "hack" has come up in the world. To hack into a computer implies not laziness but skill. In this usage, "hack" is also malicious. Hackers may be smart but they're also dangerous; they can cause crashes when they illegally intrude to alter a program.
The most common contemporary use of "hack," if I'm reading the data correctly, occurs in the recent coinage "lifehack," which is defined as "a strategy or technique adopted in order to manage one's time and daily activities in a more efficient way." I can't be the only person whose junk mail folder is regularly inundated with recommended lifehacks, most of which seem mighty silly or simplistic.
And then there's an even more specialized use. Here's one that's new to me: "scientists announced that they had genetically hacked tobacco plants to photosynthesize more efficiently." This usage seems to retain something of the older meaning of cut or slash although it is a stretch to envision a genome being attacked by a sword or a snickersnee.
Goodness gracious, it's a long road from waving at an inshoot with the old hickory to inserting bits of DNA in the tobacco genome. But here we are.
Other words of my life: slouch, cishet, yips, ramps, jot and tittle, worship, mucilage. spatchcock, umpire.