The Arabic word (Mawla) means to take charge of something and to achieve it. The author of Al-Siha says:
"The ruler is the mawla i.e. he takes charge of the country. A man is mawla i.e. he takes charge of something, for example, selling. One says that someone was the mawla -i.e. took charge- of something, and was given charge of it. For example the commander made someone take charge of something or someone was put in charge of selling something, and he took charge of the job."
The author of An-Nihayah says:
"Wilaya -guardianship- means administration with ability and action. Whoever takes charge of something becomes its Mawla -guardian-… Umar’s saying to Ali, 'You have become the mawla (guardian) of every believer,' means taking charge of every believer’s affairs."
The author of al-Qamus says:
"Wilaya -guardianship- means policy, command, and authority. Being the Mawla i.e. taking charge of something means being responsible for it. To take charge of the orphan means to be responsible for his affairs…"
The author of Lisan ul-Arab says:
"Sibawayh says, "Wilaya stands for the guardianship of someone; taking charge of his affairs and fulfilling his needs. The mawla (guardian) of a woman is he who undertakes the responsibility of contracting marriage on her behalf; she cannot get married without his agreement. Prophet Muhammad (PBUH) says: (For women who got married without the permission of their guardians, their marriage is invalid.)
Thus, the real meaning of this word (mawla) is to take charge of a matter and to carry it out. The various uses of the expression simply express this basic fact, such as saying the word 'man' for Zayd, Amr and Bakr. Allah is called Mawla because He is the ruler of the affairs of Man.
The master is addressed as mawla because he is in charge of his slave, and the slave is also called mawla because he is in charge of carrying out his master's affairs.
Likewise, neighbors, cousins, allies and sons-in-law are called mawla because they help those who need their help. So, it has a common literal meaning."
So, the meaning of Prophet’s saying (Of whomsoever I am guardian, Ali here is to be his guardian) is that over whomsoever I took charge of his affairs, Ali will take charge of his affairs. This is clear concerning the leadership, imamate and guardianship of the nation. Since the Prophet Muhammad (PBUH) was the leader, guardian and commander of the nation, so also will Ali. This is the real meaning of the word according to the opinions of the linguists. If one rejects this meaning in preference for the other manifold meanings of the word 'mawla', it would be a word that would be common in expression, but having various meanings independent from each other. In this case there would be no doubt that the meaning that agrees with the tradition would be the first one.
Some senior scholars have discussed this subject in their books. Abu Ubayda says in his book Ghareebul-Qur’an: "Mawla means worthier." He cited al-Akhtal’s poetic verse to Abdul-Malik bin Marwan as his evidence.
Al-Anbari said in his book Tafsirul-Mushkil fil-Qur’an: "Mawla means the worthier."
Az-Zajjaj and al-Farra' said, as mentioned in al-Fakhr ar-Razi’s book At-Tafsir, vol. 29 p. 227, Egyptian edition that "Mawla means worthier." It was mentioned that Abul-Abbas al- Mubarrid had said that Mawla means worthier and most deserving.
Az-Zamakhshari said in his Tafsir, vol. 4 p. 66, Egyptian edition: "In fact, Mawla means your place, where it would be better for you to be." Al-Halabi, in his book At-Taqrib, said: "Mawla, in fact, means worthier and the other expressions are derived from it. The master is a mawla because he is worthier to manage his slaves' affairs and to bear with their faults. The slave is a mawla because he is worthier to obey his master. So too are the freed slave, the helper who is more worthy of helping whom he helps, the ally to be more worthy of supporting his allies, the neighbor to be more worthy of helping his neighbor and defending him, the son-in-law to be more worthy of his relatives, the imam to be more worthy of whom he leads and the cousin to be more worthy of helping his cousins."
Since the word (Mawla) means worthier, there is no excuse to turn it away from its real meaning and seek other ones.
* Source: imamreza.net