This word has achieved a lot of prominence in the last three months, but it is being used in a way that the Oxford English Dictionary doesn’t – yet – recognise.
Shield as a verb in English goes back a long way: it appears in Beowulf. But there it is used as a transitive verb, i.e. you shield something (against or from something else). For instance an umbrella shields you from the sun. Also “It is desirable to shield against electrostatic fields without interfering in any way with magnetic fields” (slightly adapted quotation from an example from 1935 in the OED).
But now we see and hear the following sentence almost every day “People who are shielding should stay at home”. We all know what that means, but it doesn’t stop it from being a novel usage of the verb – the direct object, never stated, but always understood, is “themselves”. The NHS website says:
“If you’re at high risk (clinically extremely vulnerable) from coronavirus (COVID-19), there are extra steps you’re advised to take to protect yourself. These extra steps are called shielding.”
The use of shielding in this sentence appears to be a noun, but as the sentence is a definition of the meaning of the word, I would say that it is a metalinguistic use of it, and so no particular part of speech. Words in English are often best described not as ‘noun’, ‘verb’ etc., but as being used in nominal, or verbal, contexts. Probably the most famous example of this is the quotation from Shakespeare “But me no buts”, in which the word but, most often a conjunction, is used as both verb and noun within four words. I suggest that most laymen will not recognise this nice distinction, and that shielding has now taken on the ambiguous function of present participle and verbal noun.
The same is true of the word build, as the OED shows:
“7. quasi-passive use of the present participle, as in the house is building, originally the house is a building, where building was the verbal noun, and a = on prep. See a prep.1 12, and building n.
[1535 Bible (Coverdale) Ezra v. 16 Sence that tyme hath it bene in buyldinge.
1535 Bible (Coverdale) John ii. 20 Sixe and fourtye yeare was this temple abuyldinge.
a1665 J. Goodwin Πλήρωμα τὸ Πνευματικόv (1670) xv. 436 The Wall that is a building.]
1841 G. Catlin Lett. N. Amer. Indians II. xlvi. 95 A very pretty little town, building up.
1860 Mercantile Marine Mag. 7 300 The..lighthouse is stated to be building.
1862 G. W. Thornbury Life J. M. W. Turner I. 199 Five drawings of the abbey, then building.”
I shall be interested to read any comments – supporting or contradicting my analysis.