I love this line. It’s not one of those bottomless Shakespearean mystery lines, like “Life’s but a walking shadow” or “Our revels now are ended,” but it conveys a wonderful image while also revealing something about the character who speaks it.
Modern ears will be unaccustomed to the use of “harrows” as a verb, but the word “harrowing,” though not nearly as common as its near-synonyms “hair-raising” or “spine-tingling,” still has currency. What most people don’t realize is that the currency was minted right here, in this scene, on this line (and later, in 1.5.752, when the Ghost warns Hamlet that his account of the afterlife will “harrow up thy soul,” among other things).
Prior to Hamlet, the word “harrow” appears to have had an exclusively agricultural meaning. Originally a noun, the word refers to “A heavy frame of timber (or iron) set with iron teeth or tines, which is dragged over ploughed land to break clods, pulverize and stir the soil, root up weeds, or cover in the seed” (OED) — in other words, a thresher. The transitive verb form (which goes back at least to 1377, more than 200 years before Shakespeare) simply describes the process of using a harrow to “break up, crush, or pulverize” tough soil. If the pre-Shakespearean use of the word had any metaphorical value, it was only applied to other inanimate objects, like a ship that “harrows” the sea beneath it.
Shakespeare’s use of the word here applies it to the human soul. Now a human being can feel “broken up, crushed, or pulverized” by a powerful emotion. Horatio has no particular reason to grasp upon an agricultural metaphor, but we may assume he chooses it because all of the things you could actually do to a human being are not enough to describe the enormity of his distress.
I love this because his reaction to the Ghost is not merely fear, but rather the complete overturning of a belief system. He mentions “fear” first, but “wonder” is just as important here, and it doesn’t just have the “gee, wow, that’s cool” connotation that we tend to use today. One of its most common usages in Shakespeare’s time was “A deed performed or an event brought about by miraculous or supernatural power” (OED) — in short, a miracle. Horatio’s skepticism from mere moments past is not being shaken, nor even broken — it is being pounded into powder.
Finally, I like the line because, even as he is feeling all this, Horatio comes across as a rationalist. He describes his emotional upheaval with an almost Spock-like calm. Try shouting this line in a panic; it doesn’t work. It’s a terrific challenge for an actor, to stand stock still while his comrades are freaking out left and right, and simply declare, “This is destroying me.”