An arguable thesis is one you have to give reasons for, ..

A good thesis will be focused on your object of study (as opposed to making a big claim about the world) and will introduce the key words guiding your analysis.
To get started, you might experiment with some of these “mad libs.” They’re thinking exercises that will help propel you toward an arguable thesis.

ARGUABLE THESIS WRITING - Oracle Application Server

Of what use is a thesis that does not lead to arguments? Therefore, you must write an arguable thesis. To decide whether a thesis is arguable or not, you will have to determine if one can argue opposite to your notion. If something does not lead to a debate, it simply means that it is a fact rather than an arguable assertion. To further elaborate upon this notion, consider for instance that you topic is ‘computers have become an ideal means of transmitting information nowadays’. This statement cannot be argued. It is more of a fact. On the other hand, an arguable statement would be that ‘computers are the reason behind an increase in suicide rates nowadays’. This topic will spark debate because most people will not agree with this notion.

Facts, Opinions and Arguable Thesis Statements - SlideShare

it is arguable whether ... →

An arguable thesis is one you have to give reasons for, that is worth proving (i.e., not obvious). So my example above is not a valid thesis, because everybody knows what color the sky is. An arguable thesis might be, for example, “The sky only became blue about 1 billion years ago, when the composition of the atmosphere changed to produce the specific refraction of sunlight that makes it look blue.” (I have no idea if this is true or not, by the way—it’s just an example.) This statement is not obvious, and it would require evidence about the nature of the atmosphere a billion years ago, and explanations of why that evidence is reliable, in order to be proved.