Ok, here's the rest:

1: an act of robbery on the high seas
also : an act resembling such robbery

2: robbery on the high seas

3
a: the unauthorized use of another's production, invention, or conception especially in infringement of a copyright
b: the illicit accessing of broadcast signals

Definitions 1 & 2 absolutely do not apply here, so I initially only copied the third, because it's the only thing that could apply.

And here's the legal definition (I usually don't copy this because people tend to use the colloquial definition):

1: an act of robbery especially on the high seas
specifically : an illegal act of violence, detention, or plunder committed for private ends by crew or passengers of a private ship or aircraft against another ship or aircraft on the high seas or in a place outside the jurisdiction of any state see also air piracy

2
a: the unauthorized copying, distribution, or use of another's production (as a film) especially in infringement of a copyright software piracy
b: the unauthorized use, interception, or receipt of encoded communications (as satellite cable programming) especially to avoid paying fees for use
the statute's purpose is to proscribe the piracy of programming signals—
United States v. Harrell, 983 F.2d 36 (1993)

3: the crime of committing piracy

Again, 1 and 3 aren't relevant. 2a also doesn't apply because there's no copying or distribution.

So we're left with 2b. To decide whether it applies would require deciding whether TOS applies merely when accessing a website. I argue it doesn't, and it would require some form of pop-up or something that the user would agree to before using the service to apply. This is typically done when creating an account, but I use YouTube without an acocunt, and YouTube never prompts me to agree to any form of TOS. So the layperson's understanding here is that the content is fine to access over their app using the tools available in their app, and that any extensions in my browser should be fine since there's no reason to suspect that they wouldn't be (again, no TOS or measures on the website to prevent or discourage their use).

So the strongest argument is the legal definition 2b. There, that's the type of rebuttal I was expecting you to make, and I went ahead and did that for you, as well as providing my own rebuttal to that argument. I could cite a bunch of prior art as well (e.g. DVRs, which were totally legal when they were common), but I'm guessing you're not interested in having that discussion given your comments.

TL;DR - According to the common definition of piracy, blocking ads doesn't apply. According to the legal definition, blocking ads could apply, but that depends on whether the TOS applies and is legally enforceable.