Arbaýīn means forty. Imām Nawawi’s collection of forty ĥadīth is well known as Arbaýīn an-Nawawiyyah [Nawawi’s Quadraginta]. The Imām himself describes this in the preface:

It has been narrated to us from Álī ibn Abū Ţālib, Ábdullāh ibn Masúūd, Muáādh ibn Jabal, Abū Dardā’a, Ibn Úmar, Ibn Ábbās, Anas ibn Mālik, Abū Hurayrah, Abū Saýīd al-Khudrī rađiyAllāhu ánhum from various chains and varying wording that RasūlAllāh şallAllāhu álayhi wa sallam said: ‘Whosoever among my followers [ummatī] memorizes forty Ĥadīth related to religious knowledge shall be raised among the groups of jurists and scholars [fuqahā, úlamā] on the day of Judgement.’ in another narration: ‘Allāh táālā shall raise him a jurist, a scholar.

In the report of Abū Dardā’a: ‘I shall be his intercessor and witness on the Day of Judgement.’

In the report of Ibn Masúūd: ‘It shall be said to him: enter paradise from any of the doors you wish

In the report of Ibn Úmar: ‘He shall be listed among the group of scholars and raised among the group of martyrs

Ĥadīth masters [ĥuffāž] are agreed that this is a weak narration [đaýīf], but one with numerous routes.

Scholars – rađiyAllāhu ánhum – have written numerous treatises in this matter; the first one I know is Ábdullāh ibn Mubārak and then Muĥammad ibn Aslam at-Ţūsī, the pious master. Then, Al-Ĥasan ibn Sufyān an-Nasawī, Abū Bakr al-Ājuriyy, Abū Bakr Muĥammad ibn Ibrāhīm al-Aşfahānī, ad-Dar Quţnī, Al-Ĥākim, Abū Nuáym al-Aşfahānī, Abū Ábd ar-Raĥmān as-Sullamī, Abū Saýīd al-Mālīnī, Abū Úthmān as-Şābūnī, Muĥammad ibn Ábdullāh al-Anşārī, Abū Bakr al-Bayhaqī and many others among the earlier ones and the later ones.

I did istikhāra – prayed to Allāh táālā to help me compile a collection of forty myself, following in the footsteps of leaders and masters of this noble science. [ayimmah, ĥuffāž]

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Writing ‘forty’ in Latin makes it easy to use it as a proper noun;
because using ‘forty’ without specifying ‘what’ seems like a hanging sentence. To wit:

The Quadraginta is a magnificent book for beginners.
The Forty is a magnificent book for beginners.
The Forty Ĥadīth is a magnificent book for beginners.
The Collection of Forty is a magnificent book for beginners.

This also obviates the need to specify forty ‘what’ in other cases – like Arbaýīn of Imām Ghazāli is not just ĥadīth, even though it contains ĥadīth; we can simply say: ‘The Quadraginta’ of Imām Ghazali. It is a mind-trick, but it works.  If it helps or is easier, we can try French: ‘Quarante.’ This raises the question, why not leave it at ‘Arbaýīn’ in Arabic? Perhaps, it is because English/French are related to Latin and anything said in Latin sounds profound…Quidquid latine dictum sit, altum sonatur.