I quote Lord Neuberger in Berrisford v Mexfield Housing Co-operative Ltd (Rev 1)  UKSC 52. I grok fetter in layman terms, but this legalese noun phrase is packed with too many nouns!
- In Prudential  2 AC 386, the House of Lords overruled Midland Railway  Ch 725, effectively on the basis that a fetter of uncertain duration on the service of a notice to quit [emboldening mine] in relation to a periodic tenancy was as objectionable to the concept of a tenancy as was the existence of an uncertain term. It was not, however, suggested by Lord Templeman that Breams  2 KB 1 was wrongly decided.
- If we accept that that is indeed the law, then, subject to the point to which I next turn, the Agreement cannot take effect as a tenancy according to its terms. As the judgment of Lady Hale demonstrates (and as indeed the disquiet expressed by Lord Browne-Wilkinson and others in Prudential  2 AC 386 itself shows), the law is not in a satisfactory state. There is no apparent practical justification for holding that an agreement for a term of uncertain duration cannot give rise to a tenancy, or that a fetter of uncertain duration on the right to serve a notice to quit is invalid. There is therefore much to be said for changing the law, and overruling what may be called the certainty requirement, which was affirmed in Prudential  2 AC 386, on the ground that, in so far as it had any practical justification, that justification has long since gone, and, in so far as it is based on principle, the principle is not fundamental enough for the Supreme Court to be bound by it. It may be added that Lady Hale's Carrollian characterisation of the law on this topic is reinforced by the fact that the common law accepted perpetually renewable leases as valid: they have been converted into 2000-year terms by section 145 of the Law of Property Act 1922.
I found merely one textbook that refers to this noun phrase.
Certainly, Southward Housing Co-operative v. Walker has been criticized40 and it is true that the reasoning in Mexfield did not necessarily justify the outcome according to the intentions of the parties.'41 However, if the consequence of Mexfield is that regardless of the intention of the parties, courts will ‘inexorably’ find a tenancy for life when there is a tenancy of uncertain duration as a consequence of a fetter of uncertain duration [emboldening mine], then this has profound consequences for all landlords, not just housing co-operatives as in Mexfield.42 In particular, as Susan Pascoe argues: ‘The dual transmogrification process applied in Mexfield would be antithetical to the core essence of a periodic tenancy.’43 For these reasons, as well as the doubts as to the interpretations of the legal history in Mexfield, the result in Walker deserves consideration.
Antonia Layard, Thompson's Modern Land Law (8 edn, 2022, OUP), pp. 319n20.