1 {red dotted line on map showing route from Athens to Kiev}
2 Monty: Can't you make this flying heap go any faster?
2 Pilot: I'm redlining it!

Redlining is pushing an engine to its maximum safe operating speed. Most mechanical engineering types and roleplaying gamers will probably know this definition, but it's a little bit specialised and there are other definitions of "redlining" in fields such as loan accounting, document editing, and possibly others. So I thought a little explanatory note here might help, in case of confusion.

And if you were confused, you've now learnt something! Isn't education fun?

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2014-10-18 Rerun commentary: The link I originally had on "loan accounting" has vanished, so I have removed it. When I first reread this comic and annotation to compose a rerun comment, I imagined redlining in the context of "loan accounting" was probably something like drawing a red line on an accounts ledger to separate income from expenditure, or something similarly harmless like that. I'm not sure what sort of definition I originally linked to, but a new search brings up a Wikipedia page which explains redlining as a practice of denying people living in certain areas access to services, typically loans and mortgages, often based on discriminatory demographic profiling of the regions.

This does not sound like the sort of thing I would previously have dismissed as merely "loan accounting". The only other possibly relevant thing I could find was a reference in the book Cost Accounting for Dummies, which states that redlining is "pushing your production activity above the relevant range of production capacity".