Where does Squidward's ink come from?

[Growlie]

The few times we've seen him ink it seemed like an involuntary reflex because he was scared or angry. Maybe he can ink deliberately but doesn't bother because he knows SpongeBob and Patrick wouldn't stay away for long anyway.

Squidward's eyebrows are bushy.

 

[Fred Mercury]

it would be explained in "Time of The Month"

Spoiler

Time of the Month:
Squidward and Squilliam ink once a month and they need to find something yellow and absorbent

 

[Kukimao]

So I guess that's why he doesn't wear pants then. No point if you know you're gonna ink all over them.

 

[2000sKid]

it seems involuntary and he seems bothered by it afterwards, so I doubt it's on purpose