Bene gesserit is Latin. The only usage I know of, outside of Frank Herbert's Dune is "quamdiu se bene gesserit", which is translated as
As long as he shall behave himself well. A clause inserted in commissions, when such instruments were written in Latin, to signify the tenure by which the officer held his office.
by The 'Lectric Law Library's Lexicon. One can also find this exact quotation attributed to Bouvier's Law Dictionary.

In 1321, Thomas Stace and Thomas le Rente, bailiffs in Colchester, were deposed under the clause in the borough charter of 1200, which stated "non amoveatur quamdiu se in balliva illa bene gesserit nisi per commune consilium predictorum burgensium". Source: CHAPTER 7, "Conflict and Solidarity in Urban Politics" of The Men Behind the Masque: Office-holding in East Anglian boroughs, 1272-1460, part of a web site on Medieval English towns and © Stephen Alsford, 1998.

A state paper of 15 March 1554 in England contains

A commission (in English) from the queen to deprive John bishop of Lincoln, John bishop of Worcester and Gloucester, and John bishop of Hereford, who had been made bishops by king Edward VI with the express clause quamdiu se bene gesserit.
(Printed by Burnet, Hist. of the Reformation, vol. ii. Records to book II. number 12; Rymer, vol. xv. p. 370; quoted in Appendix III, "STATE PAPERS OF THE REIGN OF QUEEN MARY DURING THE YEARS 1553 AND 1554" in THE CHRONICLE OF QUEEN JANE AND TWO YEARS OF QUEEN MARY, "EDITED, WITH ILLUSTRATIVE DOCUMENTS AND NOTES, BY JOHN GOUGH NICHOLS, ESQ. F.S.A., PRINTED FOR THE CAMDEN SOCIETY. M.DCCC.L.")

In Matthew Henry's commentary on Zechariah 3, we read

The angel of the Lord, having taken care to make him fit for his office (and all that God calls to any office he either finds fit or makes so), invests him in it. And though he is not made a priest with an oath (that honour is reserved for him who is a priest after the order of Melchisedek, Heb. 7:21), yet, being a type of him, he is inaugurated with a solemn declaration of the terms upon which he held his office. The angel of the Lord protested to Joshua that, if he would be sure to do the duty of his place, he should enjoy the dignity and reward of it. Now see, (1.) What the conditions are upon which he enters into his office. Let him know that he is upon his good behaviour; he must walk in God's ways, that is, he must live a good life and be holy in all manner of conversation; he must go before the people in the paths of God's commandments, and walk circumspectly. He must also keep God's charge, must carefully do all the services of the priesthood, and must see to it that the inferior priests performed the duties of their place decently and in order. He must take heed to himself, and to all the flock, Acts 20:28. Note, Good ministers must be good Christians; yet that is not enough: they have a trust committed to them, they are charged with it, and they must keep it with all possible care, that they may give up their account of it with joy, 1 Tim. 6:14. (2.) What the privileges are which we may expect, and be assured of, in the due discharge of his office. His patent runs, Quamdiu se bene gesserit—During good behaviour. Let him be sure to do his part, and God will own him.

gesserit is 3rd person singular subjunctive perfect, or the 3rd person future perfect indicative, of gero, gerere, gessi, gestum, which means 'to carry, to behave or conduct oneself, to manage, to hold public office'. Sources: 501 Latin Verbs and Cassell's Latin Dictionary.

Finally, I quote this statement from a band's web page without comment:

bene gesserit is a two piece guitar band that plays dirty torch songs about death and phone booths. sweet vocals and fine guitar work from ex members of blake. they have recorded one ep.
guitar-evan clarke
guitar and singing-jim mcintyre

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