Miri = temporal power (Such as that excercised by a commander, governor, lord, prince, etc.)
It is often defined as being derived from the Persian word “miri”, which came from the Arabic “amir” which means ruler. The word however predates the Arabic additions to the Persian language, with roots going back to the earlier languages of Persia, India and the land of the five rivers.
In a recent article on Dawn.com in reference to Political unrest in Gilgit-Baltistan (once a part of the Sikh Raj during the rule of Maharaja Ranjit Singh) titled, Political unrest in Gilgit-Baltistan, the word miri is defined as “…a type of feudal system”.
The article refers to the end of the 'Colonial' rule of the Dogras, which they claim has now been replaced by the Colonial rule of Pakistan. 
Guru Hargobind is usually credited with 'coining' the use of Miri Piri to indicate that the Sikh Gurus were not just the spiritual leader of the Sikhs but also their temporal ruler as well. His adoption of the word was also meant to let the neighboring Rajas know that the Sikhs were now intending to defend themselves. Though the Sikh Gurus never attempted to establish a Sikh Kingdom or homeland, Maharaja Ranjit Singh did take over many fiefs of other Sikh and Muslim rulers. Many Sikh rulers, to avoid having him usurp their kingdoms, turned to the British for safety. Many historians have seen his conquests as an attempt to provide a homeland for the Khalsa. Whatever his reasons, the fact remains that his rule was secular. He even was careful to include members of each community (religious) as part of his court and as commanders of the Khalsa army.