Elon Musk tweeted the Elvis Presley song "Love Me Tender" a day after Twitter Inc adopted a "poison pill" to ward off the billionaire's $43 billion cash deal to acquire the social media platform.

Musk hinted at the potential of a hostile bid following his TED address on Thursday, tweeting: "It would be utterly indefensible not to put this offer to a shareholder vote."

Musk, a self-described "free speech absolutist" who has been critical of Twitter and its policies, responded to a 2017 tweet with an upside-down smiley to the question, "How much is it (Twitter)?"

Musk, the CEO of electric vehicle manufacturer Tesla Inc (TSLA.O), owns a 9.1 percent interest in Twitter and is the company's second-largest stakeholder.

Twitter shares increased 2.1 percent to $46.03 in premarket trading on Monday, remaining significantly lower than Musk's offer of $54.20 per share. Since Musk declared his interest in the company on April 4, the company's shares have increased nearly 15%.

According to persons familiar with the subject, Thoma Bravo, a technology-focused private equity group with more than $103 billion in assets under management as of December, alerted Twitter that it was studying the prospect of putting together a proposal.