Many years ago I heard my university teacher of Old English remark in passing that the phrase “dead keen” had existed in English for a thousand years, having originally been dǽd céne, “keen (i.e. brave) of deed”, and having subsequently mutated in meaning.

I’ve never heard this anywhere else, nor can I find any backing for it in the OED or elsewhere. If it had been anyone other than my teacher I wouldn’t give the notion credence for a moment. Has anyone any light to shed on this, either way?

dǽd-céne: adj. Deed-bold; agendo fortis, audax :-- Com ingán ealdor þegna, dǽdcéne mon the prince of thanes, the deed-bold man, came entering, Beo. Th. 3294; B. 1645.

Not that it proves much or answers your query, but the use of ‘dood’ (dead) as an intensifier is very common in Dutch. The list of examples in WNT is long (the words under category 2). At first glance ‘dead keen’ looks like a similar construction. The cognate of ‘keen’ is Dutch ‘koen’ still meaning brave, although it is mainly used in a poetic context nowadays. But this not the same as the use of ‘deed’ in the English constructions in question of course. The Dutch cognate of that is ‘daad’.

Edit: weaseling in “but..”.

I’m not sure that the modern phrase comes from this Old English compound, which only appears once in the OE corpus, and may be an innovation of the Beowulf poet. Compare it to dædhwæt, which makes five appearances in OE poetry. It’s more likely that the modern phrase was reinvented at a later date from the same constituent elements.

I also disagree with the existence of the OED’s first definition of keen as “wise, learned, clever.” The first cite under that definition is from the Old English translation of the Meters of Boethius:

Se wæs uðwita ælces þinges cene and cræftig, þæm wæs Caton nama. (He was a philosopher of all things keen and crafty, whose name was Cato.)

Given that there is no other Old English use in this sense, it is much more likely that the relevant sense is that of “bold, daring,” and selected for alliterative purposes. The early Middle English citations under this definition also seem to fit the “daring” sense (although I haven’t investigated them thoroughly), and the cite from 1400 would appear to fit under sense 4.a., “operating on the touch or taste like a sharp instrument,” which is a later development, but in place by 1400.

Thanks, everyone! It does look as though my teacher read dǽdcéne in Beowulf and simply jumped to an unwarranted conclusion.

So when did dead (or dood in Dutch) become an intensifier? The only concrete usage I can come up with is dead reckoning, which is still a little opaque in meaning.

Wouldn’t in the “dead of night” be such an intensifier?  Shakespeare used it: 

I am sure there are earlier and better examples…

Hmm, dubious example, in my view. Dead of night could just be a literal use of dead.

I thought, dubiously, as well. 

“dead of the night” ~= “in the quiet of the night”

but hope sprang that “dead quiet” might arise from nearby and lead to other ‘deadnesses’ of an intensifying nature.

’Dead of night’ and ‘dead quiet’ are arguably metaphorical uses. ‘Dead’ is definitely just an intensifier in phrases like ‘dead good’ or ‘pure dead brilliant’

’Dead of night’ and ‘dead quiet’ are arguably metaphorical uses. ‘Dead’ is definitely just an intensifier in phrases like ‘dead good’ or ‘pure dead brilliant’
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Or, like the man impaled on the sundial, dead on time.

Dead serious

I’ve heard of a road being described as dead straight.

And, of course, dead ringer.

Common in the UK: dead good, and dead (very) before many adjectives and adverbs.

’Dead of night’ and ‘dead quiet’ are arguably metaphorical uses.

I’m not sure about “dead quiet”, but it’s my feeling that “dead of night” is certainly metaphorical (like ”graveyard shift”). The Elizabethans took for granted that night and death were connected: in Hamlet the Ghost can only walk at night, and Hamlet says:

Tis now the very witching time of night,
When churchyards yawn and hell itself breathes out
Contagion to this world.

And in A Midsummer Night’s Dream we have:

Now it is the time of night
That the graves, all gaping wide,
Every one lets forth his sprite
In the church-way paths to glide